UNIVERSITY  OF  CALIFORNIA 
LOS  ANGELES 


PRINCIPLES  OF  METALLOGRAPHY 


10124 


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PRINCIPLES 

OF 

METALLOGRAPHY 


BY 

ROBERT  S.  WILLIAMS,  S.B.,  PH.D. 

ASSOCIATE   PROFESSOR  OP  ANALYTICAL  CHEMISTRY  AND  INSTRUCTOR  IN  METALLOG- 
RAPHY, MASSACHUSETTS  INSTITUTE   OP  TECHNOLOGY 


McGRAW-HILL  BOOK  COMPANY,  INC. 

239  WEST  39TH  STREET.    NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 

6  &  8  BOUVERIE  ST.,  E.  C. 

1920 


COPYRIGHT,  1919,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


TO  WHOSE  INSPIRATION 
MY  INTEREST  IN  METALLOGRAPHY  IS  DUE 


PREFACE 

This  little  book  has  been  written  to  meet  the  needs  of 
those  students  of  General  Science  on  Engineering  who  do 
not  specialize  in  Metallography  but  who  will  use  it  to  a 
limited  extent  in  connection  with  their  professional  work. 

It  is  hoped  that  it  will  be  of  service,  also,  to  the  general 
reader  as  an  introduction  to  an  increasingly  important 
branch  of  service  and  as  an  aid  to  the  better  under- 
standing of  the  more  highly  specialized  books. 

Greater  emphasis  has  been  laid  on  the  applications  of 
Metallography  than  on  the  physico-chemical  principles 
involved  but  it  is  believed  that  the  fundamental  ideas 
on  which  metallography  is  based  have  not  been  neglected. 

In  the  appendix  will  be  found  a  few  of  the  tables  most 
commonly  used  by  the  metallographist,  a  suggested  out- 
line of  a  brief  laboratory  course  and  a  descriptive  list  of 
the  more  important  books  and  journals  dealing  with  the 
subject. 

Thanks  are  due  to  the  authors  of  many  of  the  stand- 
ard books  on  metallography  which  have  been  freely  used 
in  the  preparation  of  this  little  volume  and  grateful 
acknowledgment  is  made  for  the  use  of  a  few  drawings 
which  have  been  copied  with  minor  changes  from  other 
books. 

Special  thanks  are  due  to  Messrs.  Bauer  and  Deiss  from 
whose  book  on  "The  Sampling  and  Chemical  Analysis 
of  Iron  and  Steel"  most  of  the  microphotographs  of 
steel  and  iron  have  been  taken. 

It  is  a  pleasure  to  express  my  appreciation  for  the 
services  of  Professor  L.  F.  Hamilton  who  has  helped 
greatly  by  his  kindly  criticism  of  the  proof. 


CONTENTS 

PAGE 
PREFACE v" 

CHAPTER 

I.  THE  SIMPLE  ALLOY  DIAGRAM 1 

II.  LABORATORY  METHODS  OF  METALLOGRAPHY 18 

III.  THE  ALLOY  DIAGRAM  AND  ITS  MEANING 39 

IV.  THE  NON-FERROTJS  ALLOYS  OF  TECHNICAL  IMPORTANCE.   ...  69 
V.  IRON  AND  STEEL 9^ 

VI.  DEFECTIVE  MATERIAL 123 

APPENDIX 136 

INDEX  .  151 


PRINCIPLES  OF 
METALLOGRAPHY 


CHAPTER  I 
THE  SIMPLE  ALLOY  DIAGRAM 

An  alloy  may  be  defined  as  the  solid  which  results 
when  two  or  more  metals  harden  from  the  molten  state. 
In  a  few  rare  cases  alloys  are  prepared  in  other  ways  as, 
for  instance,  by  compression  of  the  solid  metals  or  by 
electrolytic  deposition,  but  the  amount  of  material 
alloyed  in  this  way  is  negligible. 

Metallography,  or  Physical  Metallurgy  as  it  is  some- 
times called,  is  the  general  study  of  metals  and  alloys. 
In  its  more  restricted  sense  it  deals  with  finished  alloys, 
their  physical  and*chemical  properties,  their  internal 
structure,  the^methods  of  investigation  and,  perhaps 
most  important  of  all,  the  study  of  the  mechanical  prop- 
erties and  defects  of  the  commercial  alloys. 

When  a  pure  metal  is  melted  under  conditions  which 
make  it  possible  to  determine  the  changes  of  tempera- 
ture during  the  cooling  and  these  changes  are  indicated 
in  graphical  form,  a  curve  of  the  following  form  is  ob- 
tained (Fig.  1).  The  temperature  readings  are  taken 
at  definite  time  intervals  and  in  plotting  the  curve  these 
temperatures  are  used  as  ordinates  while  the  correspond- 
ing time  intervals  are  abscissae.  The  solidification  of 
lead  has  been  selected  as  an  example,  with  time  inter- 
vals of  ten  seconds.  It  will  be  noticed  that  the  curve 

1 


'METALLOGRAPHY 


falls  smoothly  until  the  temperature  of  327°C.  is 
reached  when  it  breaks  sharply  and  remains  horizontal 
indicating  a  constant  temperature  during  an  appreci- 
able period.  The  curve  again  falls  gradually  to  ordi- 
nary temperatures  without  further  abrupt  changes  in 
direction.  The  horizontal  line  represents  the  transition 
of  the  lead  from  the  liquid  to  the  solid  state.  The  heat 
which  is  necessary  to  maintain  the  mass  at  constant 


Time  in  .Seconds 
FIG.  1. — Cooling  curve  of  pure  lead. 

temperature  is  the  latent  heat  of  solidification.  Since 
the  amount  of  heat  liberated  at  this  time  is  proportional 
to  the  mass  of  metal  solidifying,  it  is  evident  that,  other 
conditions  being  equal,  the  length  of  the  horizontal  is 
a  measure  of  the  amount  of  material  present.  This  does 
not  hold  absolutely  in  practice  as  it  is  impossible  to  get 
ideal  cooling  conditions  but  it  is  true  to  such  a  degree 
of  approximation  that  the  fact  is  of  great  value  in  the 
thermal  study  of  alloys.  The  horizontal  has  the  addi- 
tional physical  significance  that  it  indicates  the  only 
temperature  at  which  solid  and  molten  lead  will  stay  in 


THE  SIMPLE  ALLOY  DIAGRAM 


contact  with  each  other  indefinitely.  At  any  higher 
temperature  solid  lead  will  disappear  and  at  a  lower  one 
there  will  be  no  liquid.  Several  other  methods  of 
drawing  these  " cooling  curves"  will  be  described  later 
in  connection  with  the  laboratory  study  of  the  alloys. 
The  Two  Layer  Alloy. — The  simplest  possible  alloy 
is  one  which  results  from  the  solidification  of  two  metals 


200- 


M    WSAl  15*  Al  50*Al   30  * 
10* Pb  25* Pb  50*  P6  70 * 


Time  in  Seconds 
FIG.  2. — Cooling  curves  of  aluminum,  lead  and  four  alloys. 

which  do  not  mix  even  when  they  are  in  the  molten 
condition.  In  the  strictest  sense  this  is  probably  not  a 
true  alloy  but  the  subject  deserves  consideration  because 
of  the  practical  application  of  certain  of  these  non- 
miscible  metal  pairs  in  the  form  of  solid  emulsions.  If 
aluminum  and  lead  are  mixed  in  varying  proportions,  the 
mixtures  melted,  allowed  to  cool  under  conditions  as 


4  PRINCIPLES  OF  METALLOGRAPHY 

nearly  identical  as  possible  and  temperature  readings 
are  taken  at  definite  time  intervals  during  the  cooling, 
a  series  of  curves  like  that  shown  in  Fig.  2  will  be  obtained. 
Curve  No.  1  is  the  cooling  curve  of  pure  aluminum  and 
each  successive  curve  represents  the  cooling  of  an  alloy 
containing  more  lead  than  the  one  before  it  until  pure 
lead  solidifies  as  indicated  in  Curve  No.  6.  • 

This  method  of  representing  the  temperature  changes 
in  a  series  of  alloys  is  not  satisfactory  even  in  so  simple 
a  case  and  it  becomes  impossible  in  more  complicated 
cases.  This  has  lead  to  the  use  of  a  graphical  form  of 
representation  known  sometimes  as  the  "freezing  point 
diagram"  but,  more  accurately,  as  the  equilibrium  dia- 
gram of  the  alloys.  Disregarding  for  a  moment  the 
time  taken  for  each  alloy  to  solidify,  as  shown  by  the 
horizontal  lines  in  the  series  of  curves,  it  is  evident  that 
the  temperature  changes  may  all  be  indicated  on  a  chart 
in  which  the  ordinates  represent  temperatures  and  the 
abscissae  percentage  composition  of  the  different  alloys. 
Since,  in  the  case  under  consideration,  the  melting  point 
of  neither  metal  is  affected  by  the  presence  of  the  other, 
the  freezing  point  diagram  consists  of  two  horizontal 
lines  only,  one  at  the  melting  point  of  aluminum,  the 
other  at  the  melting  point  of  lead. 

In  his  various  papers  on  Thermic  Analysis,  Tammann1 
has  shown  that,  in  addition  to  the  simple  freezing  point 
diagram  given  above,  the  introduction  of  a  curve,  or 
curves,  showing  the  time  interval  during  which  the  tem- 
perature stays  constant  for  each  alloy  of  a  series  is 
exceedingly  valuable  both  in  the  construction  and  in 
the  interpretation  of  these  equilibrium  diagrams.  The 
time  curves  are  obtained  and  applied  to  the  diagram  as 
follows:  a  straight  edge  is  applied  to  the  sloping  parts 
of  each  cooling  curve  as  shown  in  Curve  No.  2,  Fig.  2 

*Zt.  Anorg.  Chem.,  37  (1903),  303;  45  (1905),  205;  47  (1906),  289. 


THE  SIMPLE  ALLOY  DIAGRAM 


and  the  resulting  sloping  lines  are  connected  by  hori- 
zontal lines  drawn  through  the  horizontal  parts  of  the 
cooling  curves.  The  distance  between  the  points  of 
intersection  of  each  horizontal  line  with  the  two  oblique 
lines  which  it  crosses  represents  the  time  either  of  solidi- 
fication or  of  some  other  change  taking  place  at  constant 
temperature.  In  Tammann's  earlier  work  these  time 
lines  were  drawn  from  the  horizontal  temperature  lines 
to  which  they  referred  as  shown  in  Fig.  3.  This  led  to 


Al  -  658.7 


P6-327 


Composition 
FIG.  3. — Aluminum-lead  diagram. 

a  certain  amount  of  confusion  as  one  set  of  ordinates 
had  to  be  used  to  indicate  both  time  and  temperature. 
In  the  more  recent  work  of  other  metallographists  the 
time  curves  are  made  a  separate  part  of  the  diagram  as 
shown  in  Fig.  4. 

Since,  as  was  stated  on  p.  2,  the  time  taken  for  a  metal 
to  solidify  is  proportional  to  the  amount  of  material 
present  when  the  cooling  conditions  are  substantially 
identical,  it  is  obvious  that  the  time  of  solidification 
of  the  aluminum  will  be  greatest  where  its  amount  is 
greatest  and  will  decrease  to  zero  where  only  lead  is 
present.  The  triangle  which  is  formed  by  drawing  a 
line  through  the  ends  of  the  time  perpendiculars  is 


6 


PRINCIPLES  OF  METALLOGRAPHY 


useful  in  several  ways.  From  it  may  be  determined 
not  only  how  much  time  will  be  required  for  any  alloy 
in  the  series  to  solidify,  no  matter  what  its  composition, 
but  also,  what  is  far  more  important,  the  percentage 
composition  of  the  alloy.  It  is  only  necessary  to  cool 
an  equal  weight  of  the  unknown  specimen  under  standard 
conditions,  determine  the  time  interval  on  its  cooling 
curve  and  locate  the  same  time  interval  on  the  equi- 


00/U90        SO        70        GO        50        4  U        3U        ; 

o     i|o     2)0     a|o     4|o     s|o     c|o     T|O 


Composition 

FIG.  4. — Aluminum-lead  alloys  (GROYER).  The  ordinates  of  the  triangles 
marked  "time in  seconds"  correspond  to  the  horizontal  lines  in  the  curves  of 
Fig.  2. 

librium  diagram.  The  point  at  which  the  ordinate  of  the 
time  triangle  has  the  same  length  as  the  unknown  time 
interval  indicates  the  percentage  composition  of  the  alloy. 
This  is  the  fundamental  idea  of  Thermic  Analysis  and, 
while  the  results  do  not  compare  in  accuracy  with 
ordinary  quantitative  methods  nor  can  the  method  be 
used  at  all  in  many  cases,  it  is  quite  possible,  in  those 
instances  in  which  it  may  be  used,  to  get  results  very 
rapidly  indeed  and  with  a  degree  of  accuracy  which  is 
often  sufficiently  great  for  much  commercial  alloy  work. 
The  complete  equilibrium  diagram  makes  it  possible 


THE  SIMPLE  ALLOY  DIAGRAM  7 

to  predict  the  physical  condition  of  any  alloy  in  the  series 
at  any  temperature  included  in  the  diagram.  This 
question  of  the  interpretation  of  diagrams  is  one  of  the 
greatest  importance  to  the  metallographist.  Consider 
as  an  example  the  behavior  of  an  unknown  alloy  x,  Fig. 
4,  during  its  cooling  from  a  temperature  1  to  the  tem- 
perature 4.  Since  the  diagram  shows  that  in  all  mix- 
tures of  this  series  the  first  heat  evolution  occurs  at  the 
temperature  of  the  line  A  B  it  follows  that  all  alloys 
above  that  line  and,  therefore,  alloy  x  will  be  in  the  liquid 
condition  at  the  temperature  1.  When  the  tempera- 
ture has  fallen  to  that  indicated  by  A  B,  or  point  2 
on  the  dotted  line,  a  temperature  effect  is  noticed  on 
the  diagram.  Since  this  evolution  of  heat  takes  place 
at  the  same  temperature  for  all  alloys  of  the  series  and 
since  it  corresponds  to  the  melting  point  of  aluminum, 
the  assumption  is  justified  that  at  and  below  the  point 
2  the  mass  consists  of  crystals  of  aluminum  suspended 
in  a  mass  of  fused  material.  That  this  assumption  is 
correct  may  be  shown  by  removing  some  of  the  solid 
material  at  any  temperature  lower  than  that  of  point 
2  and  higher  than  point  3.  It  will  be  found  to  be 
pure  aluminum.  At  point  3  on  the  line  CD  a  second 
heat  effect  is  shown  and  since  this  point  corresponds  to 
the  melting  point  of  lead  the -same  line  of  reasoning  as 
before  shows  that  this  effect  represents  the  solidification 
of  the  lead  which  the  mixture  contained.  As  no  further 
heat  changes  are  shown  on  the  diagram  it  may  be 
assumed  that  alloy  4  is  a  solid  mixture  of  aluminum 
and  lead. 

The  diagram  as  drawn  gives  the  following  general 
information  with  regard  to  alloys  of  aluminum  and  lead. 
At  any  temperature  above  that  represented  by  AB,  any 
alloy  of  aluminum  and  lead,  no  matter  what  its  percent- 
age composition,  will  consist  of  a  mixture  of  molten 


8  PRINCIPLES  OF  METALLOGRAPHY 

aluminum  and  molten  lead  and  in  this  particular  case 
owing  to  the  great  difference  in  the  specific  gravity  of 
the  two  substances  it  will  consist  of  two  distinct  liquid 
layers.  In  the  area  between  AB  and  CD  all  alloys  con- 
sist of  a  mixture  of  solid  aluminum  and  molten  lead,  the 
relative  amounts  of  the  two  metals  varying  with  the 
percentage  composition.  In  the  temperature  range 
below  CD  the  alloy  is  made  up  of  solid  lead  and  solid 
aluminum.  Unless  the  alloy  is  vigorously  stirred  during 
the  solidification,  the  solid  will  be  found  to  consist  of 
two  distinctly  separated  layers. 

After  the  construction  of  the  diagram,  or  frequently 
in  practice  simultaneously  with  its  construction,  a  micro- 
scopic study  of  the  solid  alloys  is  made.  A  highly  pol- 
ished surface  free  from  scratches  is  obtained  on  which 
the  internal  structure  of  the  alloy  is  brought  out  by 
treatment  with  suitable  etching  reagents  which  attack 
one  constituent  more  than  the  other  and  produce  in 
this  way  depressions  in  the  surface,  color  changes  of 
the  constituent  attacked  or  other  differences  which  are 
noticeable  under  the  microscope.  The  method  of  polish- 
ing the  specimens  and  preparing  them  for  microscopic 
examination  will  be  considered  in  the  section  on  Labora- 
tory Methods,  p.  18.  The  following  microphotograph 
(Fig.  5)  of  a  lead-aluminum  alloy  shows  the  way  in  which 
the  microscope  is  used  to  confirm  the  results  obtained  by 
temperature  measurements.  As  was  to  be  expected 
from  the  diagram  the  microscope  shows  two  distinct 
layers,  the  dark  lead  at  the  bottom  and  the  lighter  colored 
aluminum  above  it. 

Plastic  Bronze. — The  most  important  example  of  the 
alloys  formed  by  non-miscible  metals  is  the  class  of 
copper-lead  alloys  known  as  plastic  bronzes,  much  used 
as  bearing  metals.  Melted  lead  forms  an  emulsion  with 
melted  copper  and  if  the  cooling  of  the  alloy  takes  place 


THE  SIMPLE  ALLOY  DIAGRAM  9 

rapidly  enough  the  lead  will  be  found  more  or  less  uni- 
formly distributed  throughout  the  mass  of  copper  in  the 
form  of  spherical  drops.  By  the  addition  of  nickel  or 
other  high  melting  material  in  small  amounts  it  is  pos- 
sible to  prepare  an  alloy  containing  50  per  cent,  lead 
though  the  usual  plastic  bronzes  contain  only  from  15 
to  30  per  cent.  lead.  In  alloys  of  this  type  the  copper 
gives  the  necessary  strength  while  the  lead  increases  the 
plasticity  of  the  alloy  and  acts  as  a  lubricant. 


FIG.  5. — Aluminum-lead  alloy   (75  X)- 

While  it  is  probably  true  that  no  two  metals  are 
absolutely  insoluble  in  each  other,  either  in  the  liquid 
or  in  the  solid  state,  the  solubility  in  the  case  just  de- 
scribed is  so  slight  that  for  practical  purposes  it  may  be 
disregarded.  There  are  many  alloys  of  this  class  but 
those  of  copper  and  lead  are  the  only  ones  of  technical 
interest. 

The  Eutectic  Alloy. — The  next  type  of  alloy  to  be 
considered  is  that  in  which  the  two  metals  are  completely 
miscible  in  the  liquid  state  and  completely  non-miscible, 
or  insoluble  in  each  other,  in  the  solid  state.  It  is  a 


10 


PRINCIPLES  OF  METALLOGRAPHY 


well-known  fact  that  in  most  cases  in  which  one  sub- 
stance is  added  to  another,  in  which  it  will  dissolve, 
the  freezing  point  of  the  solvent  is  lowered.  An  illus- 
tration of  this  phenomenon  is  the  preparation  of  the 
ice-salt  freezing  mixture  which  is  able  to  produce  a  tem- 
perature twenty-one  degrees  below  that  of  ice  alone. 
Numerous  alloys  behave  in  the  same  way.  One  of  the 
best  known  of  these  is  the  lead-antimony  alloy  which  has 
many  important  commercial  uses.  If  a  small  amount  of 
lead  is  added  to  molten  antimony  the  freezing  point 


FIG.  6. — Lead-antimony  cooling  curves. 

of  the  latter  is  lowered  to  a  considerable  extent  and 
increasing  quantities  of  lead  still  further  lower  the  freez- 
ing point.  If,  on  the  other  hand,  a  small  amount  of 
antimony  is  added  to  pure  lead  the  melting  point  of  the 
lead  is  also  lowered  and,  as  in  the  case  of  the  antimony, 
is  progressively  lowered  by  the  addition  of  greater 
quantities  of  antimony.  The  effect  of  the  addition  of 
each  metal  to  the  other  is  shown  in  the  series  of  curves 
of  Fig.  6.  It  is  obvious  that  since  each  metal  lowers  the 
freezing  point  of  the  other,  the  lines  connecting  these 
freezing  points  must  intersect  at  some  point  as  shown 
by  the  dotted  lines  in  the  drawing  Fig.  6.  This  point 


THE  SIMPLE  ALLOY  DIAGRAM 


11 


of  intersection  is  one  of  great  interest  and  importance 
and  has  been  called  the  eutectic  point.  The  alloy  cor- 
responding to  the  composition  at  which  the  two  lines 
intersect  is  the  eutectic  alloy  and  the  temperature  is  the 
eutectic  temperature.  If  the  data  given  by  the  cooling 
curves  is  assembled  in  the  form  of  an  equilibrium  diagram, 
as  before,  the  diagram  takes  the  form  shown  in  Fig.  7. 
The  significance  of  this  type  of  diagram  can  be  under- 
stood most  readily  by  considering  the  physical  changes 
which  take  place  in  a  few  special  cases  as,  for  instance, 


50  30 

Composition 
FIG.  7. — Lead-antimony    diagram. 


10    Sb 


during  the  cooling  of  alloys  1,  2,  and  3  in  Fig.  7.  Since 
the  V-shaped  curve  was  obtained  by  connecting  the 
freezing  points  of  the  separate  alloys,  it  is  evident  that 
the  area  above  the  V  represents  a  temperature  range  in 
which  everything  is  in  the  molten  condition.  This  is  fre- 
quently called  the  liquidus.  As  the  temperature  of  alloy 
1  falls  no  change  takes  place  until  the  line  PbB  is  reached, 
at  which  temperature  pure  lead  begins  to  separate.  The 
result  of  the  separation  is  to  leave  a  solution  richer  in 
antimony  than  the  original  solution  and,  therefore,  one 
which  has  a  lower  freezing  point.  Pure  lead  continues 


12  PRINCIPLES  OF  METALLOGRAPHY 

to  separate  with  the  consequent  formation  of  solutions 
increasingly  rich  in  antimony  and  therefore  with  lower 
melting  points.  The  fact  that  the  solution  from  which 
the  lead  is  crystallizing  is  of  constantly  changing  compo- 
sition is  the  reason  for  the  shape  of  the  cooling  curves 
(see  Fig.  6)  in  alloys  of  this  type.  The  curve  is  not  a 
horizontal  line  as  in  the  case  of  a  pure  metal  but,  as  it 
represents  an  infinite  number  of  freezing  points,  it  ap- 
pears in  the  form  of  a  change  in  the  direction  of  the 
normal  curve,  or  as  a  "  hold  "  as  it  is  often  called.  As  the 
liquid  from  which  the  lead  is  separating  becomes  richer 
in  antimony  it  approaches  the  eutectic  composition  in- 
dicated by  B.  Since  this  represents  the  lowest  possible 
temperature  at  which  lead  and  antimony  alloys  can 
solidify,  it  is  evident  that  when  the  residual  liquid  finally 
reaches  the  eutectic  composition  the  metal  will  solidify 
at  this  constant  temperature.  The  same  reasoning 
applies  to  alloy  3  except  that  in  this  case  the  antimony 
crystals  separate  first.  The  primary  separation  of 
antimony  is  followed  by  an  enrichment  of  the  remaining 
liquid  with  lead  until  the  eutectic  composition  is  reached 
again.  At  the  composition  2  no  change  takes  place  until 
the  eutectic  temperature  is  reached  when  lead  and  anti- 
mony solidify  together  in  the  form  of  the  eutectic  mix- 
ture. The  line  DBE  (the  eutectic  line)  represents  that 
temperature  below  which  the  alloy  is  solid  and  is  there- 
fore called  the  solidus  line. 

To  summarize  the  statements  made  above,  it  may  be 
said  that  any  alloy  having  a  composition  between  D 
and  B  shows  two  heat  evolutions  on  cooling,  one  which 
corresponds  to  the  primary  separation  of  lead  and  a 
second  which  is  due  to  the  solidification  of  the  eutectic 
mixture.  The  amount  of  residual  material  having  the 
eutectic  composition  is  greater  the  nearer  the  composi- 
tion approaches  that  of  the  point  B  and  is  zero  at 


THE  SIMPLE  ALLOY  DIAGRAM  l 

point  D.  Along  SbB  antimony  is  the  primary  separa- 
tion and  the  eutectic  mixture  of  antimony  and  lead  is 
the  secondary. 

Because  of  the  constant  temperature  at  which  the 
eutectic  separates  it  was  formerly  believed  that  the 
eutectic  was  a  compound.  The  microscope  shows  that 
this  is  not  the  case  but  that,  on  the  contrary,  the  eutectic 
alloy  is  an  extremely  intimate  mixture  of  the  two  com- 
ponent metals.  Since  the  eutectic  is  a  mixture  of  the 
two  metals  and  since,  as  shown  in  the  diagram,  the  eutec- 
tic line  extends  from  one  side  of  the  diagram  to  the  other, 
it  follows  that  while  lead  and  antimony  are  wholly  mis- 
cible  and  soluble  in  each  other  in  the  liquid  state  they 
are  wholly  non-miscible,  or  insoluble  in  each  other,  in 
the  solid  state.  That  the  metals  are  insoluble  in  each 
other  in  the  solid  state  must  be  true  as  the  diagram  shows 
that,  however  small  an  amount  of  either  metal  is  added 
to  the  other,  there  is  always  the  secondary  heat  effect 
at  the  eutectic  temperature. 

The  practical  application  of  the  time  curves  is  much 
more  evident  from  this  diagram  than  from  the  preceding 
one.  As  the  time  taken  for  the  eutectic  to  solidify  is 
greatest  where  there  is  most  eutectic,  it  follows  that  the 
time  curve  has  its  maximum  at  the  eutectic  composi- 
tion and  drops  to  zero  at  the  pure  metals.  This  fact 
is  most  useful  in  the  construction  of  the  diagram  of  two 
unknown  metals.  Formerly  the  location  of  the  eutec- 
tic point  was  a  matter  of  repeated  trials  with  no  advance 
information  as  to  the  probable  location  of  the  point. 
With  the  introduction  of  the  time-line  idea  the  question 
is  much  simplified.  It  is  only  necessary  to  determine  the 
time-lines  for  a  few  alloys.  After  plotting  these  lines 
on  a  horizontal  base  the  ends  are  connected.  The  in- 
tersection of  the  two  oblique  lines  resulting  gives  a  close 
approximation  to  the  eutectic  composition  so  that  its 


14 


PRINCIPLES  OF  METALLOGRAPHY 


exact  determination,  if  that  is  desired,  is  a  matter  of  a 
very  few  additional  experiments. 


FIG.  8. — Lead-antimony  alloy  with  excess  lead.     Corresponds  to  @,  Fig.  7 

(75  X)     (HOMERBERG). 


FIG.  9. — Lead-antimony  eutectic.     Corresponds  to  @.  Fig.  7  (75  X) 

(HOMERBERG) . 

The  microscopic  structure  of  these  alloys  is  exactly 
what  would  be  predicted  from  the  diagram.     All  alloys 


THE  SIMPLE  ALLOY  DIAGRAM 


15 


from  pure  lead  to  the  eutectic  composition  show  pri- 
mary lead  crystals  surrounded  by  more  or  less  of  the 


FIQ.  10. — Lead-antimony   alloy   with    excess   antimony.      Coi responds   to 

®,  FlG.  7       (HOMEKBEBG). 


FIG.   106. — Bismuth-tin.     Eutectic  with  slight  excess  tin     (SAWYER). 

eutectic   B,    depending   on   the   composition    (Fig.    8). 
Alloys  from  the  antimony  side  to  the  eutectic,  show 


16 


PRINCIPLES  OF  METALLOGRAPHY 


primary  antimony  crystals  imbedded  in  the  eutectic 
(Fig.  10).  The  alloy  having  the  composition  B  shows 
simply  the  fine-grained  eutectic  structure  without  pri- 
mary crystals  of  either  metal  (Fig.  9).  A  better  defined 
eutectic  is  shown  in  Fig.  106,  the  Bi-Sn  eutectic. 

The  lead-antimony  alloys  are  commercially  of  much 
importance.  Antimony  is  too  brittle  to  be  of  use  alone 
but  because  of  its  hardness  it  gives  to  lead  properties 
which  are  very  desirable  for  certain  purposes.  The  alloys 
are  used  for  acid  proof  coatings,  for  type  and  for  light 
bearings. 

The  eutectic  alloys  of  greatest  commercial  importance 


Composition 
FIG.  11. — Lead-tin  diagram. 

are  in  the  tin-lead  series  of  which  solders  are  made. 
The  incomplete  diagram,  Fig.  11,  shows  those  changes 
which  are  of  technical  interest  but  omits  certain  facts 
which  will  be  considered  later.  Of  the  many  mixtures 
used,  two  characteristic  types  are  tin  solder,  containing 
about  37  per  cent,  lead  (corresponding  to  the  eutectic 
composition)  and  plumber's  solder  with  approximately 


THE  SIMPLE  ALLOY  DIAGRAM  17 

67  per  cent.  lead.  The  diagram  shows  that  between  the 
points  A  and  B,  corresponding  to  a  temperature  drop 
of  about  70°,  the  alloy  consists  of  lead  crystals  supported 
in  a  molten  metal.  This  produces  the  pasty  consistency 
which  makes  the  plumbers  " wiped  joint"  possible. 
The  two  examples  just  given  illustrate  in  a  general 
way  the  factors  which  are  commonly  determined  in  the 
study  of  alloys.  Many  other  types  of  alloy-  diagrams 
have  been  worked  out  which  deal  with  the  formation  of 
intermetallic  compounds,  solutions  of  one '  metal  in 
another,  transitions  of  one  compound  into  another, 
either  during  the  solidification  of  the  alloy  or  after  it 
has  completely  solidified,  and  other  possible  changes 
which  may  take  place.  Before  considering  these  more 
complex  alloy  diagrams  and  their  applications  in  prac- 
tical work  it  will,  perhaps,  be  simpler  to  study  some  of 
the  methods  and  forms  of  apparatus  that  are  actually 
used  in  the  laboratory  preparation  and  microscopic 
study  of  the  alloys. 


CHAPTER  II 
LABORATORY  METHODS  OF  METALLOGRAPHY 

The  preceding  chapter  indicates  that  in  the  laboratory 
study  of  the  alloys  and  in  the  construction  of  their 
diagrams,  one  of  the  most  important  factors  to  consider 
is  the  succession  of  heat  changes  which  take  place  when 
the  molten  mixture  of  metals  passes  into  the  completely 
solid  state.  In  a  few  cases,  notably  in  the  cooling  of 
steel,  changes  of  vital  importance  occur  far  below  the 
point  of  solidification  of  the  alloy  so  that  it  is  necessary 
in  special  instances  to  follow  the  cooling  to  very  low 
temperatures.  A  study  of  these  thermal  changes  nec- 
essarily involves  three  factors:  (1)  A  method  of  melting 
the  mixed  metals,  (2)  a  container  in  which  the  metals 
can  be  melted,  and  (3)  an  apparatus  for  measuring  tem- 
peratures and,  especially,  temperature  changes. 

1.  Furnaces. — The  furnaces  used  in  the  melting  of 
metals  vary  so  much  with  the  amount  of  material  to  be 
melted  and  with  the  melting  points  of  the  metals  in- 
volved that  but  one  furnace  will  be  described  in  detail. 
There  is  no  difficulty,  however,  in  building  or  buying 
small  furnaces  for  any  sort  of  alloy  work. 

For  the  study  of  the  heat  changes  which  take  place 
in  alloys  of  low  melting  point  (90Q°C.  or  lower)  a  con- 
venient form  of  apparatus  is  shown  in  the  photograph 
Fig.  12  and  given  in  detail  in  the  sketch  Fig.  13.  A  is  an 
iron  tube  about  5  inches  long  and  from  2^  to  4  inches 
in  diameter.  It  can  be  made  quite  readily  by  threading 
one  end  of  a  short  piece  of  steel  pipe  and  screwing  on  a 
cap.  The  open  end  is  fastened  to  a  triangle  of  heavy 

18 


LABORATORY  METHODS  OF  METALLOGRAPHY         19 


FIG.  12. — Photograph  of  a  simple  type  of  cooling  curve  apparatus. 


Thermo-Couple 


opper  Leads 
to  MilU:Voltme)e 


Milli-voltmeter 


Fio.  13. — Diagram  sketch  of  melting  system  and  cooling  curve  apparatus. 


20  PRINCIPLES  OF  METALLOGRAPHY 

iron  or  chromel  wire  so  that  the  tube  may  be  suspended 
over  a  burner  by  means  of  a  ring  and  stand.  The  source 
of  heat  may  consist  of  one  or  more  Tirrell  or  Meker 
burners  or,  for  very  high  temperatures,  a  blast  lamp. 
The  flame  is  protected  from  draught  by  means  of  a 
concentric  cylinder  of  asbestos  cloth  separated  from  the 
outer  wall  of  the  iron  tube  by  a  space  of  ^  to  1  inch 
through  which  the  gas  flame  can  pass.  This  asbestos 
collar  also  retains  the  heat  and  makes  the  melting  proc- 
ess easier  and  quicker.  This  simple  method  of  heating 
may  be  replaced  by  any  of  the  more  elaborate  types  of 
furnaces.  The  more  expensive  wire  wound  electric  re- 
sistance furnaces  of  the  vertical  type  (dental  furnace 
type)  are  much  easier  to  control  if  provided  with  a  vari- 
able resistance.  For  the  high  melting  elements  like 
nickel,  cobalt,  chromium  and  the  metals  of  the  platinum 
group  a  carbon  resistance  furnace  is  necessary,  the  details 
of  construction  of  which  may  be  found  in  any  book  on 
applied  electricity.  A  recently  developed  electric  fur- 
nace of  the  induction  type  promises  to  be  of  great 
value  in  the  study  of  the  high  melting  alloys. 

2.  Container  for  the  Melted  Metals. — The  material 
and  shape  of  the  container  depend  on  the  temperature 
at  which  the  alloys  melt,  the  properties  of  the  melted 
metal  and  the  shape  of  the  furnace.  For  use  with  the 
furnace  just  described  a  tube  of  the  shape  shown  in  B, 
Fig.  13,  is  most  convenient.  This  tube  is  embedded 
in  sand  in  the  iron  case  A  so  that  heat  may  be  distributed 
to  it  as  uniformly  as  possible.  For  alloys  with  a  low 
melting  point  (less  than  700°C.)  hard  glass  tubes  are  most 
useful  as  the  filling  of  the  tube  and  adjusting  of  the  tem- 
perature measuring  instrument  can  be  watched.  For 
alloys  which  melt  above  the  softening  point  of  glass, 
tubes  of  porcelain  or  fused  quartz  are  required.  Thin- 
walled  tubes  of  unglazed  porcelain  may  be  used  for 


LABORATORY  METHODS  OF  METALLOGRAPHY         21 

almost  all  metals.  In  a  few  instances  where  the  metal 
forms  an  oxide  which  is  highly  reactive  chemically  (e.g., 
chromium  oxide  or  manganese  oxide),  it  may  be  necessary 
to  use  the  much  more  fragile  magnesia  tubes.  This 
expedient,  fortunately,  is  seldom  needed.  Tubes  of 
this  size  limit  the  amount  of  alloy  to  be  studied  to  about 
40  grams.  When  larger  weights  of  metal  are  to  be  used 
larger  furnaces  and  crucibles  must  be  substituted. 

Filling  the  Tube  or  Crucible. — The  metallic  elements 
of  which  the  alloy  is  to  be  composed  are  weighed  in  the 
desired  proportions  in  the  form  of  small  chips,  clippings 
or  drillings  and,  in  most  cases  in  laboratory  practice, 
are  mixed  before  they  are  introduced  into  the  tube.  In 
exceptional  cases  it  may  be  necessary  to  melt  the  less 
volatile  component  first,  adding  the  more  volatile  ele- 
ment in  successive  small  portions.  The  amount  of 
material  to  be  used  is  determined  by  the  accuracy  de- 
sired in  the  final  results.  As  a  laboratory  experiment 
or  as  a  preliminary  survey  of  the  field  to  determine  the 
general  shape  of  the  equilibrium  diagram,  20  to  30 
grams  of  the  mixture  is  enough.  It  must  be  clearly 
emphasized,  however,  that  where  great  accuracy  is  re- 
quired and  slight  heat  changes  are  to  be  looked  for,  the 
amount  of  material  must  be  very  greatly  increased, 
often  up  to  400  to  500  grams.  As  the  metals  used  should 
be  of  great  purity  and  are  therefore  expensive  it  is  sel- 
dom desirable  to  experiment  with  such  large  amounts  of 
material. 

Stirring. — There  is  often  a  marked  tendency  for  the 
molten  metals  to  separate  into  layers,  especially  if  they 
differ  considerably  in  specific  gravities  and  mix  with  diffi- 
culty or  not  at  all.  In  such  cases  the  liquid  mixture 
must  be  stirred  during  the  course  of  the  experiment. 
This  can  usually  be  done  by  means  of  a  glass  or  porce- 
lain rod  having  a  circular  bend  at  the  bottom  through 


22  PRINCIPLES  OF  METALLOGRAPHY 

which  the  thermometer  or  other  temperature  measur- 
ing device  can  pass  (see  C,  Fig.  13).  This  rod  is  moved 
slowly  up  and  down  during  the  cooling  of  the  melted 
alloy. 

Oxidation  in  the  Tube. — It  is  necessary  in  all  cases  to 
protect  the  metals  during  the  melting  and  during 
the  solidification  from  the  oxidizing  effect  of  the  air. 
In  a  few  instances  this  may  be  done  by  covering  the 
surface  with  powdered  charcoal  but  it  is  usually  more 
convenient  and  more  effective  to  melt  the  metals  in  an 
inert  gas.  Hydrogen,  carbon  dioxide  and  nitrogen  have 
all  been  used  for  this  purpose.  The  gas  passes  from  the 
generator  into  the  melting  tube  through  the  bent  glass  or 
porcelain  tube,  Fig.  13.  As  a  safety  measure,  if  hydro- 
gen is  used,  it  is  best  to  pass  the  gas  through  a  small 
drying  tube  containing  a  number  of  disks  of  wire  gauze. 
The  gauze  will  cool  the  gas  so  that  there  is  no  danger  of 
setting  fire  to  the  hydrogen  in  the  generator  or  storage 
tank. 

The  oxidation  effects  are  slight  with  the  low  melting 
metals  protected  in  this  wray  so  that  it  may  be  assumed 
that  the  composition  of  the  resulting  alloy  is  the  same 
as  the  composition  of  the  mixture  from  which  it  was 
made.  This  is  not  the  case  with  high  melting,  easily 
oxidized  metals,  even  under  the  most  favorable  condi- 
tions, so  that  the  final  composition  of  the  alloy  should 
be  determined  by  chemical  analysis. 

Weight  and  Atomic  Per  Cent. — In  all  industrial  alloy 
work,  and  for  most  laboratory  purposes,  the  metals  are 
mixed  according  to  their  percentages  by  weight  of  the 
total  amount  of  material  used.  It  will  be  shown  later 
that  some  metals  form  intermetallic  compounds.  In 
this  case  a  system  based  on  the  atomic  relationships  is 
more  convenient.  For  example,  a  compound  of  tin  and 
magnesium  containing  70.95  per  cent,  tin  by  weight 


LABORATORY  METHODS  OF  METALLOGRAPHY         23 

is  known.  This  percentage  composition  gives  no  indica- 
tion of  the  relation  of  the  atoms  in  the  compound.  If 
now  the  composition  is  indicated  in  atomic  per  cent., 
it  will  be  found  to  be  33.33  atomic  per  cent,  tin  and  66.66 
atomic  per  cent,  magnesium,  showing  at  once  that  the 
formula  is  SnMg2.  The  following  expression  shows  the 
method  of  converting  weight  per  cent,  into  atomic  per 
cent.  : 

A  =  atomic  weight  of  first  metal; 

B  =  atomic  weight  of  second  metal; 

p  =  weight  per  cent,  of  A; 

q  =  weight  per  cent,  of  B. 
Then: 

Atomic  per  cent,  of  A  =  -  — 


and: 

100  q~ 
Atomic  per  cent.  x)f  B  =  —     —  r 


3.  Measurement  of  Temperature  Changes.1  —  A  mer- 
cury thermometer  will  serve  for  the  measurement  of 
the  temperature  changes  with  the  very  low  melting 
alloys  as  those  of  sodium,  potassium  or  the  amalgams. 
Almost  all  alloy  work,  however,  requires  higher  tempera- 
tures than  can  be  determined  in  this  way  and  some  form 
of  pyrometer  must  be  used.  As  in  the  case  of  the  fur- 
naces, many  excellent  pyrometers  of  different  sorts  are 
obtainable.  One  of  these  pyrometers  will  be  described 
as  it  illustrates  the  general  method  of  use  which  may  be 
applied  with  slight  variations  to  any  of  the  other  instru- 
ments. Protected  from  direct  contact  with  the  molten 

1  For  a  detailed  description  of  temperature  measurements  see  "The 
Measurement  of  High  Temperatures"  by  BUKGESS  and  LE  CHATLIEB. 


24  PRINCIPLES  OF  METALLOGRAPHY 

metal  by  a  glass,  quartz  or  porcelain  tube  E  (Fig.  13), 
are  two  wires  of  different  metals  joined  at  the  bottom 
to  form  the  thermal  junction  or  thermoelement.  For  the 
measurement  of  low  temperatures  one  of  these  wires 
may  be  copper,  the  other  constantan  (a  copper-nickel 
alloy).  For  somewhat  higher  temperatures,  a  thermal 
couple  made  of  chromel-iron  may  be  used.  For  tem- 
peratures up  to  about  1650°C.,  a  rare  metal  couple, 
one  wire  of  which  is  platinum,  the  other  an  alloy  either 
of  platinum  and  rhodium  or  platinum  and  iridium,  is 
required.  The  two  wires  forming  the  thermal  couple  must 
be  insulated  from  each  other  inside  the  protecting  tube. 
This  may  be  done  by  wrapping  one  wire  with  asbestos 
thread  or  by  incasing  it  in  short  lengths  of  capillary 
tubing  of  quartz  or  porcelain.  The  loose  ends  of  the 
wire  pass  into  a  jar  F  where  they  are  kept  either  at 
constant  temperature  or  at  some  temperature  which 
may  be  determined  from  time  to  time  and  will  not  vary 
more  than  two  or  three  degrees  during  the  experiment. 
A  convenient  arrangement  is  to  have  these  wires  pass 
through  a  cork  stopper  into  a  Thermos  or  other  vacuum 
walled  bottle.  If  ice  is  kept  in  the  container  during  the 
run,  no  correction  for  the  temperature  of  the  "cold  junc- 
tion" is  needed.  Otherwise,  a  correction  for  the  tem- 
perature of  the  cold  junction  must  be  made.  The  cold 
ends  of  the  thermocouple  are  connected  inside  the  jar  F 
to  insulated  copper  wires  leading  to  the  instrument  G. 
The  instrument  on  which  the  temperatures  are  read  is 
commonly  a  millivoltmeter.  Most  of  the  millivoltmeters 
recommended  for  temperature  work  of  this  character  are 
provided  with  two  scales,  one  of  which  reads  millivolts 
and  the  other  temperatures  directly,  either  in  Centigrade 
or  Fahrenheit  degrees.  The  temperature  scale  may  be 
used  with  only  a  single  pair  of  metals  and  even  then  the 
reading  varies  somewhat  with  continued  use  of  the  couple, 


LABORATORY  METHODS  OF  METALLOGRAPHY         25 

so  that,  except  for  commercial  work,  the  millivolt  scale 
is  almost  always  used  in  spite  of  the  obvious  advantages 
of  a  direct  temperature  reading. 

In  order,  then,  to  establish  the  connection  between 
the  millivoltage  as  read  and  the  temperature  to  which 
it  corresponds,  calibration  is  necessary.  This  can  be 
most  simply  done  by  determining  the  melting  points 
of  a  series  of  pure  substances  and  constructing  a  plot, 
using  the  known  temperatures  as  ordinates  and  the 
instrument  readings  as  abscissae.  The  following  list  of 
pure  substances  and  melting  or  boiling  points  is  given  for 
convenience  but  any  materials  having  definite  melting 
points  at  suitable  intervals  may  be  used. 

Degrees  C. 

Water  b.  p 100 

Tinm.  p '...     231.9 

Sulphur  b.  p 444.5 

Antimony  m.  p 630 

Silver  m.  p 960. 2 

Copper  m.  p ' 1082.8 

In  locating  the  freezing  points  of  the  substances  used 
in  the  calibration,  it  is  convenient  to  follow  the  same 
procedure  that  is  followed  later  in  studying  the  alloys 
themselves.  The  pure  substance,  tin  for  example,  is 
heated  until  it  is  completely  melted.  The  supply  of 
heat  is  then  cut  off  and  the  melted  mass  allowed  to  cool 
slowly.  Temperature  readings  are  now  taken  at  defi- 
nite time  intervals.  If  the  temperature  is  high  and  is 
falling  rapidly,  intervals  of  five  seconds  are  allowed  be- 
tween each  consecutive  reading  of  the  millivoltmeter 
scale.  When  the  cooling  rate  is  normal  (about  5  to  10 
degrees  per  minute)  an  interval  of  ten  seconds  between 
readings  will  show  any  material  changes  in  the  cooling 
rate.  At  low  temperatures  where  the  difference  in 
temperature  between  the  alloy  and  its  surroundings  is 
small,  the  cooling  will  be  so  slow  that  a  much  less  frequent 
reading  of  the  instrument  is  required.  The  frequency  of 


26  PRINCIPLES  OF  METALLOGRAPHY 

readings  is  a  question  of  judgment  but  ten  seconds  be- 
tween readings  may  be  taken  as  a  reasonable  interval. 
If  the  millivoltages  are  now  plotted  as  ordinates  and  the 
time  intervals  as  abscissae,  a  curve  will  be  obtained  show- 
ing a  horizontal  break  at  that  millivoltage  corresponding 
to  the  freezing  point  of  the  material  in  question.  This 
horizontal  line  which  indicates,  not  only  the  temperature 
of  solidification  but  the  time  taken  for  the  material  in 
question  to  solidify,  varies  in  length  with  the  amount  of 
material  used  and  with  its  latent  heat  of  fusion.  The 
constancy  of  this  time  interval  under  the  same  condi- 
tions make  its  determination  of  great  value  in  making 
the  alloy  diagrams. 

Various  types  of  millivoltmeters  are  in  use,  some  direct 
reading,  some  of  the  mirror  type  in  which  the  deflection 
is  magnified  by  reflecting  a  beam  of  light  on  a  scale  at 
some  distance  from  the  instrument  and  still  others  of 
the  recording  type.  Whichever  instrument  is  used, 
the  general  method  of  study  consists  in  establishing  a 
connection  between  the  temperature  changes  and  the 
intervals  of  time  during  which  these  changes  occur. 
Instead  of  using  a  millivoltmeter  as  a  temperature  meas- 
uring instrument,  a  potentiometer  may  be  used.  This 
instrument  requires  somewhat  more  care  and  experi- 
ence in  operation  than  the  millivoltmeter  but  the  results 
are  far  more  accurate.  Where  exact  temperatures,  rather 
than  relative  temperatures  or  rapid  temperature  changes 
are  required,  the  potentiometer  should  always  be  used. 
The  modern  forms  of  potentiometer  can  be  operated  so 
much  more  rapidly  than  the  early  types  that  their  use 
by  metallographists  is  constantly  increasing. 

Having  considered  the  various  factors  involved  in 
the  study  of  the  temperature  changes  taking  place  when 
an  alloy  is  cooled  it  may  be  helpful  to  summarize  these 
factors  in  a  brief  description  of  an  actual  melting  opera- 


LABORATORY  METHODS  OF  METALLOGRAPHY         27 

tion.  Referring  once  more  to  Fig.  13  (p.  19),  the  proc- 
ess is  as  follows.  Metals  X  and  Y  are  weighed  in  a 
finely  divided  condition,  such  weights  of  the  metals 
being  taken  as  will  produce  an  alloy  of  the  required  com- 
position and  in  such  amounts  that  with  the  apparatus 
described  the  total  weight  will  be  between  20  and  30 
grams.  These  are  mixed  in  the  tube  B,  imbedded  in 
sand  in  the  furnace  tube  A.  The  stirring  rod  C,  the  gas 
intake  tube  and  the  thermoelement  protector  E  pass 
through  a  cork  stopper  or  a  suitably  perforated  brass 
cap.  The  insulated  wires  pass  to  a  constant  temperature 
bottle  F  and  then,  by  copper  leads,  to  a  millivolt-meter 
or  potentiometer  G.  Heat  is  applied  to  the  sand  bath 
and  the  metals  melted,  allowing  the  introduction  of  the 
thermocouple  tube  and  the  stirring  rod.  The  heat  is 
then  shut  off  and  the  temperature  instrument  is  read 
at  definite  time  intervals  until  allj  the  heat  changes 
have  taken  place  and  the  hard  alloy  is  cooling  at  a  uni- 
form rate.  The  readings  are  carefully  recorded.  In  the 
study  of  a  binary  (two  component)  alloy  this  operation 
is  repeated  with  a  series  of  metal  mixtures  of  varying 
composition.  If  the  nature  of  the  equilibrium  curve  is 
wholly  unknown,  mixtures  are  generally  taken  which 
vary  in  composition  by  10  per  cent,  intervals  from  one 
pure  metal  to  the  other.  These  eleven  points  (this 
includes  the  melting  points  of  the  constituent  metals) 
will  usually  indicate  the  general  shape  of  the  diagram, 
after  which  the  necessary  number  of  additional  mixtures 
can  be  selected  for  study  in  the  vicinity  of  the  more 
essential  points  such  as  eutectics,  intermetallic  com- 
pounds or  other  characteristic  features  suggested  by  the 
preliminary  survey.  In  simple  cases  a  very  few  addi- 
tional mixtures  will  give  all  the  information  necessary, 
while  in  the  more  complex  alloys  40  or  50  points  are 
sometimes  needed  to  establish  the  diagram. 


28 


PRINCIPLES  OF  METALLOGRAPHY 


Plotting  the  Cooling  Curves.— Various  methods  of 
plotting  the  experimental  results  are  in  use.1  Of  these 
methods  the  simple  time-temperature  curve  is  used  more 
frequently  than  any  other  although  the  " inverse  rate" 
curve  has  decided  advantages  when  the  heat  effects 


dT 

~3T 


FIG.  14. — Inverse    rate    cooling    curve.     (T     =    temperature,    t     =    time.) 

are  small.  With  this  latter  method  the  ordinate  rep- 
resents temperature  as  in  the  simple  curve  but  the 
abscissa  is  the  reciprocal  of  the  rate  of  cooling,  that  is, 
the  time  necessary  for  the  temperature  to  fall  through  a 
definite  small  interval  (5°  to  10°  for  example).  The 
resulting  curve  has  the  form  shown  in  Fig.  14  in  which 
t  =  time  in  seconds  and  T  =  temperature  in  degrees. 
Constructing  the  Diagram. — After  plotting  the  in- 
dividual cooling  curves,  a  sheet  of  coordinate  paper  of 
suitable  size  is  selected  and,  using  percentage  composi- 
tions as  abscissae  and  temperature  holds  on  the  cooling 
curves  as  ordinates,  a  diagram  is  constructed  as  indi- 
cated in  Chapter  I.  The  relative  length  of  any  hori- 

1See  GULLIVER,  "Metallic  Alloys,"  Ed.  2,  p.  175. 


LABORATORY  METHODS  OF  METALLOGRAPHY         29 

zontal  lines  that  may  be  found  in  the  set  of  cooling 
curves  is  next  determined  and  these  " time-lines"  are 
also  plotted  as  an  independant  but  closely  associated 
part  of  the  diagram. 

Preparation  and  Microscopical  Examination  of  the 
Polished  Alloys. — For  the  examination  of  the  ordinary 
alloy,  a  specimen  having  a  surface  about  ^  square  inch 
is  large  enough.  In  special  cases  of  defective  material, 
as  for  instance,  broken  rails  or  similar  articles,  much 
larger  pieces  are  needed.  The  small  specimen  for  polish- 
ing is  first  obtained  as  a  cubical  or  cylindrical  piece  by 
cutting  it  from  the  larger  piece  with  a  hack  saw,  or  a 
power  saw  if  available.  Where  the  material  is  too  hard 
to  cut,  as  in  the  case  of  Duriron  and  similar  high  silicon 
alloys,  the  specimen  is  broken  with  a  hammer  and  a 
fragment  of  suitable  size  selected  for  polishing.  If  only 
a  very  small  piece  of  metal  is  available,  it  can  be  handled 
by  placing  it  in  a  molten,  readily  fusible  alloy  and  allow- 
ing the  whole  mass  to  solidify.  The  tiny  fragment  can 
then  be  polished  with  the  larger  mass  of  metal  in  which 
it  is  imbedded. 

The  next  step  in  the  preparation  of  the  specimen  con- 
sists in  filing  the  surface  to  be  examined  or  grinding  on  a 
carborundum  wheel.  If  the  surface  is  originally  very 
rough,  two  or  more  wheels  may  be  used  to  advantage, 
each  finer  than  the  one  preceding.  The  subsequent  opera- 
tion of  polishing  consists  in  rubbing  the  specimen  on 
abrasive  materials  of  increasing  fineness  until  a  scratch- 
free,  mirror-like  surface  is  obtained.  In  polishing  the 
specimen  it  should  be  turned  through  ninety  degrees  each 
time  the  change  is  made  from  one  polishing  surface  to 
the  next.  The  rubbing  on  each  abrasive  surface  should 
be  continued  until  the  scratches  produced  by  the  next 
coarser  surface  (perpendicular  to  the  direction  of  polish- 
ing) have  been  removed.  The  number  of  polishing 


30  PRINCIPLES  OF  METALLOGRAPHY 

surfaces  needed  in  the  preparation  of  a  specimen  de- 
pends on  the  hardness  of  the  material.  With  hard 
steel  specimens  a  number  of  grades  of  abrasive  material 
are  needed  each  but  little  finer  than  the  one  preceding  it, 
while  with  soft  alloys  like  those  of  lead  or  tin  very  few 
intermediate  grades  of  polishing  material  are  required. 

For  the  average  specimen  the  following  sequence  of 
abrasives  will  serve:  coarse  file,  fine  file,  very  fine  emery 
wheel,  emery  papers  (French  emery  "Marke  Hubert" 
is  best)  1C,  IF,  0,  00,  000  and  0000.  In  using  the  finer 
grades  of  emery  paper,  from  00  down,  a  drop  of  oil  should 
be  applied  to  the  surface  of  the  specimen.  From  this 
point  the  abrasion  is  carried  out  by  finely  divided  pow- 
ders used  in  the  form  of  aqueous  suspensions  which  are 
sprinkled  from  time  to  time  on  the  finest  quality  of 
broadcloth  or  chamois  skin.  An  excellent  series  of 
polishing  powders  has  been  prepared  especially  for  metal- 
lographic  purposes  by  the  Norton  Alundum  Company 
of  Worcester.  Three  powders  that  will  answer  for  most 
purposes  are  60  minute  emery  or  carborundum,  followed 
by  "alundum  F"  and  finally  by  "levigated  alumina." 
Many  metallographists  prefer  jeweler's' rouge  as  a  final 
polishing  agent  rather  than  alumina.  Rouge  gives  a 
brilliant  final  polish  but  has  the  disadvantages:  (1) 
that  it  tends  to  cause  a  slight  flowing  of  the  surface  metal, 
especially  with  the  softer  alloys;  and  (2)  that  it  is  a  dirty 
and  unpleasant  abrasive  with  which  to  work. 

Polishing  may  be  done  by  hand  if  necessary  but  the 
operation  is  a  long  and  tiring  one  so  that  machine  polish- 
ing is  to  be  recommended  if  an  apparatus  is  available. 
For  hand  polishing,  the  various  grades  of  paper  and  the 
cloths  which  are  to  be  used  as  bases  for  the  polishing 
liquids  are  tacked  to  smooth  boards  (3  inches  X  8 
inches  is  a  convenient  size). 

Many  excellent  polishing  machines  are  now  obtain- 


LABORATORY  METHODS  OF  METALLOGRAPHY         31 

able.  They  differ  somewhat  in  detail  but  all  consist 
in  general  of  rotating  disks  of  wood  or  metal  over  which 
the  abrasive  paper  or  cloth  is  stretched.  Machines  hav- 
ing disks  revolving  in  a  horizontal  plane  are  more  con- 
venient to  use  than  those  of  the  vertical  type. l  The  liquid 
suspension  of  abrasive  powder  maybe  applied  to  the  cloth 
by  means  of  a  camels  hair  brush,  by  a  wash  bottle  or  by  the 
simple  shaking  bottle  suggested  in  the  sketch,  Fig.  15. 


FIG.  15. — Flask  for  suspended  abrasives. 

Scrupulous  care  in  the  use  of  polishing  papers  and 
powders  must  be  taken  to  avoid  the  transfer-  of  a  coarse 
abrasive  to  a  finer  surface.  A  single  particle  of  grit  on 
the  final  polishing  wheel  may  injure  an  otherwise  perfect 
specimen  so  that  repolishing  is  necessary. 

Etching  is  a  selective  chemical  action  which  will 
effect  one  constituent  of  the  alloy  more  than  the  other 
and  its  purpose  is  to  develop  the  internal  structure  of 
the  polished  alloy.  The  number  of  etching  solutions 
which  have  been  used  is  very  large  and  varies  from 
simple  reagents  like  dilute  acids  to  extremely  complex 
mixtures  for  special  purposes.  The  special  etching 
agents  used  in  the  examination  of  brass  and  bronze  and  of 
steels  will  be  considered  later  in  connection  with  the 
study  of  these  important  technical  alloys.  For  general 

1  A  detailed  description  of  mechanical  polishing  devices  will  be  found 
in  SATTVEUR,  "Metallography  and  Heat  Treatment  of  Iron  and  Steel," 
Ed.  2,  p.  54  et  seq. 


32  PRINCIPLES  OF  METALLOGRAPHY 

purposes  dilute  acids  (2  per  cent,  nitric  or  hydrochloric) 
or  dilute  alkalis  (sodium  hydroxide  or  ammonium  hy- 
droxide) are  used.  One  of  the  most  successful  general 
reagents  is  made  by  dissolving  10  grams  of  ferric  chloride 
(FeCh)  in  100  cc.  of  alcohol.  It  will  be  realized  that 
etching  is  a  chemical  problem  so  that  the  particular 
reagent  used,  as  well  as  its  strength,  depends  on  the 
chemical  solubility  of  the  components  of  the  alloy. 
For  this  reason  specific  directions  cannot  be  given.  In 
most  cases,  however,  the  problem  is  one  which  is  readily 
solved  by  a  few  trials  if  the  effect  of  the  reagent  is 
watched  under  the  microscope.  Etching  is  most  uni- 
form if  the  specimen  is  immersed  face  upward  in  a 
shallow  dish  containing  the  reagent.  The  formation 
of  gas  bubbles  on  the  surface  must  be  prevented  by  keep- 
ing the  dish  in  constant  rocking  motion  or  by  swabbing 
the  exposed  surface  with  a  bit  of  absorbent  cotton  at  fre- 
quent intervals.  After  etching,  the  surface  is  washed 
with  water  and  with  alcohol  and  is  dried  either  with 
cotton  or  by  a  warm  blast  of  air.  Perfectly  dried  speci- 
mens may  be  kept  in  a  desiccator  for  a  long  period  with- 
out change.  Specimens  may  be  preserved  indefinitely 
by  coating  them  with  cellulose  acetate. 

For  etching  spots  on  a  large  surface  or  as  a  rapid 
method  for  determining  the  most  suitable  etching  agent 
for  a  given  alloy,  a  small  swab  of  cotton  moistened  with 
the  reagent  may  be  used. 

The  Microscope.1 — The  polished  and  etched  surface 
of  the  specimen  must  be  examined  by  reflected  light. 
If  a  low  power  objective  is  used  (magnifying  less  than 
25  diameters),  a  beam  of  light  can  be  reflected  on  the 
surface  in  an  oblique  direction,  passing  below  the  objec- 
tive. As  the  magnification  increases  the  distance  between 
the  objective  and  the  surface  becomes  so  small  that 

1  See  SAUVEUR,  "  Metallography  and  Heat  Treatment  of  Iron  and  Steel, 
Ed.  2,  p.  67. 


LABORATORY  METHODS  OF  METALLOGRAPHY 


33 


this  method  of  illumination  is  impossible.  In  such 
cases  the  microscope  must  be  provided  with  a  "  vertical 
illuminator, "  the  principle  of  which  is  shown  in  Fig.  16. 
The  beam  of  light,  coming  from  a  small  arc  lamp,  a 
nitrogen  filled  tungsten  or  other  powerful  light  source, 
passes  into  the  side  arm  of  the  illuminator  and  is  reflected 
by  a  prism  or  plate  down  to  the  surface  of  the  speci- 


FIQ.   16. — Sketch  of  vertical  illuminator  (sheet  glass  type)  dotted  lines  show 
the  direction  of  rays  reflected   from   the  specimen. 

men,  illuminating  a  spot  on  the  surface  so  intensely 
that  it  can  not  only  be  examined  but  photographed. 
Such  an  illuminator  can  be  fitted  to  any  microscope  but, 
where  much  metallographic  work  is  to  be  done,  one  of 
the  regular  metal  microscopes  will  be  found  much  more 
convenient  and  effective.  The  magnifications  most 
commonly  used  are  5x,  lOx,  50x,  75x,  lOOx  and  200x. 
In  rare  cases,  with  extremely  fine-grained  structure,  a 
magnification  of  500x  or  even  lOOOx  may  be  necessary. 
As  a  matter  of  permanent  record  and  for  purposes  of 
comparison  of  samples  from  different  sources,  photo- 
graphs of  the  etched  surfaces  are  often  made  and  all 


34  PRINCIPLES  OF  METALLOGRAPHY 

modern  metal  microscopes  are  provided  with  a  photo- 
graphic attachment. 

Photographing  Metal  Specimens.— The  beam  of  light 


A. — The  complete  metallographic  camera. 


B. — Enlarged  section  showing  the  illuminator  and  microscope. 
FIG.  17. — The  Leitz  metal  microscope. 

from  the  polished  specimens  can  be  reflected  to  the  eye- 
piece for  visual  examination  or  it  may  pass  through  the 


LABORATORY  METHODS  OF  METALLOGRAPHY         35 

bellows  of  a  camera  to  a  sensitive  plate.  The  preceding 
cuts,  Fig.  17  a  and  b,  show  the  arrangement  of  the  Leitz 
metallographic  microscope.  It  is  very  similar  to  other 
metal  microscopes  and  illustrates  the  general  principles 
of  them  all. 

Plates. — For  making  microphotographs  the  chief  req- 
uisites of  good .  plates  are  color  sensitiveness  and  fine 
grain.  With  steel  samples  in  which  the  surface  of  the 
specimen  is  light,  with  black  or  gray  markings,  the  in- 
expensive " Stanley"  plates  will  be  found  satisfactory. 
For  colored  specimens,  like  brasses  and  bronzes  for 
example,  a  more  color-sensitive  plate  has  to  be  used. 
The  "Wratten  M"  plate  gives  excellent  results.  It  has 
the  disadvantage  that  it  cannot  be  handled  near  the 
ordinary  dark  room  light  but  must  be  manipulated 
either  in  absolute  darkness  or  by  a  special  "safe  light." 
"  Standard  Orthonon"  plates,  also  made  by  the  Eastman 
Company,  are  effective  both  for  steel  and  for  brass  photo- 
graphs. The  author  has  found  the  "Wellington  Ortho 
Process"  plates  very  satisfactory  for  all  purposes  and 
they  can  be  handled  without  danger  near  a  fairly  bright 
ruby  light.  In  photographing  colored  metal  surfaces  a 
color  screen  of  some  sort  and  a  piece  of  ground  glass  are 
generally  placed  between  the  illuminating  lamp  and  the 
specimen  to  be  photographed.  The  color  screen  in- 
creases the  contrast  between  the  components  while  the 
ground  glass  is  to  diffuse  the  light  and  prevent  the  reflec- 
tion of  the  hot  carbons  of  the  arc  on  the  photographic 
plate.  A  yellow-green  screen  gives  excellent  results  in 
photographing  the  yellow  alloys  of  copper. 

Exposure  of  the  Photographic  Plate. — The  time  of 
exposure  may  vary  from  a  few  seconds  to  ten  or  fifteen 
minutes  depending  on  the  source  of  light,  the  nature  of 
the  specimen,  the  color  of  the  screen  and  the  kindx>f  plate 
used.  The  operator  soon  recognizes  these  factors  so 


36  PRINCIPLES  OF  METALLOGRAPHY 

that  he  can  estimate  the  length  of  exposure  with  con- 
siderable accuracy.  When  the  conditions  are  entirely 
unknown  the  following  test  method  will  save  time  and 
plates.  Expose  as  usual  for  a  few  seconds.  Instead  of 
shutting  off  the  light,  as  would  ordinarily  be  done,  push 
in  the  opaque  screen,  which  is  used  to  cover  the  plate  in 
the  holder,  about  one-half  inch  (^  in.),  in  order  to  shut 
off  a  portion  of  the  exposed  plate.  After  another  short 
interval  push  in  the  screen  another  half  inch.  Repeat 
this  operation  until  the  opaque  screen  has  been  pushed 
into  place  in  the  holder.  When  the  plate  is  developed 
it  will  show  a  series  of  bands  each  of  which  represents 
an  exposure  for  a  somewhat  longer  time  than  the  one 
preceding  it.  Select  the  correctly  exposed  strip  from 
the  banded  negative,  record  its  time  of  exposure  and  use 
substantially  the  same  length  of  exposure  for  photographs 
taken  under  similar  conditions. 

Development  of  the  Exposed  Plates. — Directions 
for  the  preparation  of  developing  solutions  will  be  found 
in  each  box  of  plates.  Tray  development  requires 
much  experience  for  complete  success  so  that  the  use  of  a 
developing  tank  is  strongly  recommended.  The  opera- 
tion of  the  tank  is  simple  and  the  results  are  positive  and 
uniform.  Of  the  many  kinds  of  developer  on  the  market 
it  is  probable  that  pyro-soda  will  give  the  best  results  for 
metallographic  work.  After  the  development  is  com- 
pleted, the  plates  are  placed  in  saturated  "hypo" 
(sodium  thiosulphate)  solution  until  the  white  opaque 
coating  has  dissolved  and  the  negatives  have  become  trans- 
parent. The  clear  negatives  are  then  washed  in  run- 
ning water  for  an  hour,  after  which  they  are  placed  in  a 
drying  rack  and  allowed  to  become  perfectly  dry  and  hard. 

Printing,  Finishing  and  Mounting. — Photomicrographs 
require  the  greatest  possible  detail  in  printing  so  that  a 
glossy  printing  paper  is  commonly  used.  Glossy  Velox, 


LABORATORY  METHODS  OF  METALLOGRAPHY         37 

Glossy  Cyko  or  other  papers  of  the  same  sort  give  excel- 
lent results.  Printing  directions  are  always  inclosed 
with  the  paper.  After  the  prints  are  washed  a  brilliant 
finish  is  obtained  by  placing  them  face  down  on  a  ferro- 
type plate.  To  keep  them  from  sticking  to  the  surface  of 
the  ferrotype  plate  it  must  be  cleaned  before  use  with  a 
solution  of  beeswax  in  benzol  or  by  a  prepared  cleaner 
such  as  the  Ingento  Polishing  Compound.  After  the 
prints  are  in  position  on  the  plate  the  surplus  water  is 
removed  by  means  of  a  print  roller  (squeegee)  and  they 
are  then  allowed  to  dry.  The  dry  prints  are  then 
mounted  on  suitable  cards  for  examination  and  filing. 
Dry  mounting  tissue  is  excellent  for  this  purpose. 
Plates,  paper,  mounting  tissue,  etc.,  can  be  purchased  of 
any  dealer  in  photographic  supplies.  If  many  photo- 
graphs are  to  be  taken  printed  cards  of  the  general  style 
shown  in  Fig.  18  will  be  found  almost  indispensable. 

In  addition  to  the  thermic  and  microscopic  methods 
of  alloy  study,  other  properties  of  alloys  are  occasion- 
ally used  in  determining  their  constitution.  Among 
these  less  important  properties  are  electrical  and  heat 
conductivity,  heat  expansion,  and  magnetic  effects.  An 
excellent  discussion  of  the  connection  between  these 
different  properties  and  the  equilibrium  diagram  will 
be  found  in  Desch,  "  Metallography,"  Ed.  2,  p.  230. 

In  closing  this  chapter  on  the  methods  of  alloy  study 
it  must  be  stated  that  the  mechanical  testing  of  the 
metals  for  tensile  strength,  hardness,  elongation  and 
other  physical  properties  is  of  great  and  constantly  in- 
creasing importance  to  the  practical  metallographist. 
A  discussion  of  this  phase  of  the  subject  is  beyond  the 
scope  of  this  book  and  the  reader  is  referred  to  Rosen- 
hain,  "  Physical  Metallurgy,"  in  which  the  methods  of 
mechanical  testing  are  considered  in  detail. 


463286 


38 


PRINCIPLES  OF  METALLOGRAPHY 


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CHAPTER  III 
THE  ALLOY  DIAGRAM  AND   ITS   MEANING 

In  Chapter  I  the  construction  of  two  simple  alloy 
diagrams  was  outlined.  The  first  was  the  diagram 
representing  the  melting  of  two  metals  which  do  not 
dissolve  in  each  other  either  in  the  liquid  or  solid  state. 
The  second,  usually  referred  to  as  the  eutectic  diagram, 
represented  graphically  the  behavior  of  two  metals 
which  do  dissolve  completely  in  each  other  in  the  liquid 
(molten)  state  but  are  wholly  insoluble  in  the  solid  state.1 

The  Solid  Solution.^— A  third  relationship  which  may 
exist  between  metals  is  a  partial  or  complete  solubility 
of  one  metal  in  the  other  in  the  solid  state  as  well  as  when 
the  metals  are  molten.  This  relation  is  known  as  the 
formation  of  " solid  solution"  or,  less  correctly,  " mixed 
crystals."  The  solid  solution  differs  from  the  liquid 
solution  simply  in  its  physical  condition.  Like  a  liquid 
solution  it  is  perfectly  homogeneous  and  may  be  un- 
saturated  or  saturated.  Metal  A  may  retain  10  per 
cent,  of  metal  B  in  the  solid  state  but  if  an  attempt  is 
made,  to  add  12  per  cent,  of  B  to  molten  A  the  solid  which 
separates  on  cooling  is  a  saturated  solid  solution  of  10 
per  cent.  B  in  A.  The  excess  B  remains  in  the  liquid  to 
separate  later  as  part  of  a  eutectic  mixture,  as  a  constit- 
uent of  an  intermetallic  compound,  or  in  some  other 
form.  This  conception  of  the  solid  solution  of  two 
metals  as  wholly  analogous  to  the  liquid  solution  makes 
the  graphical  representation  simple. 

1  It  is  probably  not  true  that  any  two  metals  are  wholly  insoluble 
in  each  other  in  the  solid  state  but  in  many  cases  the  solubility  is  so 
small  that  it  escapes  detection. 


.40 


PRINCIPLES  OF  METALLOGRAPHY 


Consider  as  an  example  the  diagram  of  the  alloys  of 
copper  and  silver  (Fig.  19).  This  differs  from  the  simple 
eutectic  diagram  shown  in  Fig.  7  (p.  11)  only  in  the 
location  on  it  of  the  lines  Ag-«  and  Cu-£,  the  significance 
of  which  is  merely  that  molten  silver,  in  which  copper 
has  been  dissolved,  is  able  to  retain  about  6  per  cent,  of 
it  after  the  silver  has  solidified  and  that  molten  copper, 
in  its  turn,  is  capable  of  retaining  an  equal  amount  of 
silver  in  the  solid  state.  When  an  alloy  of  silver  and 


Fio    19. — Copper-silver  diagram    (HEYCOCK  AND   NEVILLE,    FREDRICK    AND 
LEROUX)  . 

copper  containing  less  than  29  per  cent,  copper  is  al- 
lowed to  solidify,  the  crystal  which  first  separates  is  not 
the  pure  element,  as  in  the  case  of  the  separation  of 
lead  on  cooling  lead-antimony  alloys,  for  example,  but 
is  a  homogeneous,  crystalline  solution  of  copper  in  sil- 
ver. The  behavior  of  solid  solutions  during  the  actual 
solidification  process  will  be  considered  shortly.  The 
significance  of  the  diagram,  then,  is  as  follows.  Any 
alloy  of  silver  and  copper  containing  less  than  6  per  cent, 
of  copper,  on  the  one  hand,  or  less  than  6  per  cent,  of 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING  41 

silver  on  the  other,  will  solidify  as  an  unsaturated  solid 
solution  which  is  perfectly  homogeneous  and  under  the 
microscope  shows  only  the  fine  polyhedral  lines  character- 
istic of  a  single  crystalline  solid  (Fig.  8,  p.  14).  If 
now  the  amount  of  copper  added  to  the  silver  is  ever  so 
slightly  in  excess  of  6  per  cent.,  the  solid  alloy  is  no  longer 
homogeneous  but  shows  a  second  structure  element,  in 
this  case  the  eutectic  E.  It  is  evident,  then,  that  in  the 
range  between  the  points  a  and  /3,  the  diagram  is  exactly 
analogous  to  the  simple  eutectic  of  lead  and  antimony, 
the  only  difference  being  that  with  silver  and  copper 
alloys  the  constituents  of  the  eutectic  are  not  the  pure 
metals  as  with  lead  and  antimony  but  are  the  saturated 
solid  solutions  a  and  j3.  The  extent  of  solubility  of  the 
metals  in  each  other  may  be  determined  in  two  ways. 
First,  since  there  is  no  separation  of  a  second  crystal 
form  until  the  concentrations  represented  by  points  a 
and  /3  are  exceeded,  it  follows  that  no  evolution  of  heat 
will  be  noticed  on  the  cooling  curve  at  the  eutectic  tem- 
perature (778°C.)  until  the  limit  of  solubility  has  been 
passed.  Or,  conversely,  if  a  horizontal  line  is  drawn 
through  the  points  locating  the  eutectic  temperature, 
the  ends  of  the  line  will  be  found  at  points  a  and  /3,  the 
limits  of  the  solid  solution.  The  second  method  of 
locating  the  limit  of  solubility  in  the  solid  state  is 
microscopic  examination.  The  microscopic  appearance 
of  the  copper-silver  alloys  is  exactly  what  would  be 
expected  from  the  diagram.  From  100  per  cent,  silver 
to  94  per  cent,  silver  the  solids  are  homogeneous.  From 
94  per  cent,  silver  to  71.9  per  cent,  the  solid  alloys  show 
gradually  decreasing  quantities  of  the  solid  solution  a 
imbedded  in  the  eutectic  E.  From  71.9  per  cent,  to  6 
per  cent,  silver  the  solid  alloy  shows  gradually  increasing 
amounts  of  the  solid  solution  0  imbedded  in  the  same 
eutectic  E  and  from  6  per  cent,  silver  to  pure  copper 


42  PRINCIPLES  OF  METALLOGRAPHY 

the  alloys  are  once  more  homogeneous.  When  the 
limit  of  solubility  is  reached,  the  addition  of  the 
slightest  excess  of  either  metal  to  the  other  forms  an 
alloy  which  is  no  longer  homogeneous  under  the 
microscope. 

The  silver-copper  alloy  is  one  of  great  commercial 
importance  as  it  is  the  alloy  of  which  silver  coins  are 
made.  American  coins  and  those  of  several  European 
countries  contain  90  per  cent,  silver  and  10  per  cent, 
copper.  British  coinage  is,  however,  slightly  richer  in 
silver,  containing  92.5  per  cent.1  Several  alloys  of  the 
same  type  are  known,  among  them  aluminum-tin,  bis- 
muth-tin, cadmium-tin,  but  except  for  the  silver-copper 
and  aluminum-tin,  none  are  of  technical  importance. 
Aluminum  with  a  low  percentage  of  tin  has  been  used  to 
a  limited  extent  in  making  light  castings  but  is  unsatis- 
factory because  of  its  ready  corrosion. 

The  solubility  of  one  metal  in  another  may  increase, 
as  shown  in  the  series  of  diagrams  in  Fig.  20,  until  the 
condition  indicated  in  D  is  realized.  The  line  a-p, 
representing  the  secondary  or  eutectic  separation,  has 
gradually  shortened  with  the  increase  in  the  mutual 
solubility  of  the  two  metals  until  in  the  alloy  D  the  line 
has  disappeared  wholly,  showing  that  the  metals  are 
soluble  each  in  the  other  in  all  proportions.  Diagrams 
A  and  D  differ  radically  in  this  respect  that,  although 
both  show  the  two  metals  to  be  soluble  in  each  other  in 
ail. proportions  in  the  liquid  state  and,  also,  that  each 
metal  lowers  the  melting  point  of  the  other,  diagram  A 
indicates  that  the  two  metals  are  completely  insoluble 
in  each  other  in  the  solid  state  while  diagram  D  shows 
that  the  metals  are  completely  soluble  in  each  other  in  the 
solid  state.  Alloy  E  in  diagram  A  is  as  inhomogeneous 
as  possible  while  aft  in  diagram  D  is  perfectly  homogene- 

^LAWj  "Alloys  and  Their  Industrial  Applications,"  Ed.  2,  p.  27?. 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING 


ous  and  represents  simply  that  one  of  the  series  of  perfect 
solutions  which  has  a  melting  point  lower  than  any  of  the 
other  solid  solutions  in  the  series.  This  is  often  referred 
to  as  the  solid  solution  minimum. 

It  is  worth  while  to  consider  briefly  the  way  in  which 
such  a  solid  solution  is  formed  during  the  cooling  of  the 


C  D 

FIG.  20. — Development  of  the  solid  solution. 

liquid  metal.  The  relations  between  copper  and  man-, 
ganese  are  shown  in  Fig.  21.  When  an  alloy  containing 
62  per  cent,  manganese  and  38  per  cent,  copper  is  at  the 
temperature  represented  by  the  point  z  it  is  completely 
liquid.  When  the  point  6  is  reached  the  alloy  begins 
to  solidify.  The  fact,  however,  which  distinguishes 
between  the  cooling  of  an  alloy  which  is  to  -form  a  solid 


44 


PRINCIPLES  OF  METALLOGRAPHY 


solution  and  a  simple  eutectic  alloy  is  that  the  crystal 
which  first  separates,  in  this  case  at  point  6,  is  not  a 
pure  metal  but  is  a  solid,  the  composition  of  which  is 
represented  by  the  point  ft1.  That  the  crystal  bl  which 
separates  from  the  molten  metal  at  b  actually  differs 
from  the  liquid  in  its  composition  may  be  determined  by 
the  removal  of  some  of  the  crystals  after  the  alloy  has 


Afn 


Cu 


p|Cu    9|0          8|0  7|0        6|0          5lO         4|0  3|0        2|0 


Fro.  21. — Copper  manganese  alloys. 

partially  solidified.  The  crystals  and  the  remaining 
metal  are  then  analyzed.  In  the  case  of  Mn-Cu,  the 
crystals  will  be  found  considerably  richer  in  manganese 
than  the  liquid  from  which  they  are  removed.  As  the 
mass  cools  the  composition  of  the  crystals  changes  along 
the  line  bibv,  while  the  liquid  varies  in  composition  along 
the  line  bblv.  Referring  to  these  conditions  in  terms  of 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING  45 

equilibrium  it  is  said  that  crystal  bl  is  in  equilibrium 
with  liquid  b,  crystal  bm  with  liquid  6n  and  crystal  6V 
with  liquid  6IV.  In  other  words  at  the  temperature  rep- 
resented by  the  line  b  lbm  crystals  of  6IH  will  stay 
unchanged  indefinitely  in  a  liquid  of  the  composition  6n. 
This  stable  relationship  between  crystal  and  melt  would 
not  exist  between  b11  and  6IH  if  the  temperature  was 
changed  in  the  slightest  degree.  Nor,  if  the  tempera- 
ture was  kept  constant  could  any  manganese-rich  liquid 
and  solid  of  different  compositions  than  those  corre- 
sponding to  6n  and  b111  exist  together.  The  fact  that 
61,  6nl  and  bv  represent  solids  in  equilibrium  with  liquids 
6,  611  and  6IV  respectively  has  given  the  name  solidus 
curve  to  the  line  connecting  the  points  ft1,  b111,  bv  and 
liquidus  curve  to  the  line  6,  b11,  biv  connecting  those 
points  which  represent  the  composition  of  the  liquids 
with  which  these  solids  are  in  contact. 

It  is  necessarily  true  that  the  crystal  which  solidifies 
last  (6V)  must  have  exactly  the  same  composition  as  the 
original  liquid  melt  (6).  The  perpendicular  line  b-bv 
connecting  these  two  points  is  one  of  great  importance 
in  interpreting  the  diagram,  as  it  makes  it  possible  to 
state  the  relative  quantities  of  liquid  and  solid  that  can 
exist  together  at  any  given  temperature  within  the 
solidification  range.  It  is  obvious  that  the  amount  of 
crystal  b1  existing  in  the  presence  of  liquid  b  is  infinitely 
small  and  equally  obvious  that  the  amount  of  liquid  blv 
existing  in  contact  with  the  crystal  bv  is  also  infinitely 
small.  Between  these  extremes  lie  an  infinite  number  of 
relationships  between  solid  and  liquid.  These  relation- 
ships, however,  are  graphically  represented  by  the  hori- 
zontal distances,  left  and  right,  from  the  line  6,  bv  to 
any  pair  of  liquid  and  solid  phases  in  the  series.  As  a 
specific  illustration,  it  is  true  that  the  quantity  of 
crystal  b111  is  to  the  quantity  of  liquid  6TI  inversely  as  the 


46  PRINCIPLES  OF  METALLOGRAPHY 

distances  from  these  points  to  the  line  b,  bv  or  as  ob11 
is  to  oft111. 

The  nature  of  the  solidification  of  a  solid  solution 
crystal  naturally  determines  the  shape  of  its  cooling 
curve.  When  a  pure  metal  or  a  eutectic  mixture  solidi- 
fies, the  crystal  separating  and  the  liquid  from  which  it 
separates  have  the  same  composition  and,  following  well 
established  laws,  the  temperature  stays  constant  during 
freezing  and  appears  as  a  horizontal  line  on  the  tempera- 
ture-time curve.  In  the  freezing  of  solid  solutions,  how- 
ever, the  solidifying  crystal  changes  its  composition 
constantly  as  does  the  solution  from  which  it  is  crystalliz- 


Time  in  seconds 
FIG.  22. — Type  of  solid  solution  curve. 

ing.  The  solidification,  then,  is  not  that  of  a  single 
solution  but  of  an  infinite  number  of  solutions  and  the 
solutions  formed  have  a  corresponding  number  of  melt- 
ing points.  The  effect  of  this  sort  of  freezing  is  to  pro- 
duce a  curve  which  shows  not  a  horizontal  freezing  line, 
but  an  oblique  line  indicating  a  change  in  the  normal 
cooling  rate  during  the  solidification  interval.  The 
normal  curve  of  a  solid  solution  alloy  is  shown  in  Fig. 
22  in  which  point  b  represents  the  beginning  of  the  freez- 
ing and  point  B  the  end,  after  which  the  normal  cooling 
rate  is  resumed.  The  perpendicular  distance  between 
b  and  B  is  shown  by  the  line  bbv  and  represents  the  tem- 
perature drop  during  the  process  of  solidification.  Point 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING  47 

6,  then,  represents  one  point  on  the  liquidus  curve,  Fig. 
21,  and  B,  or  6V  a  point  on  the  solidus  curve.  By  con- 
structing a  number  of  freezing  point  curves  and  determin- 
ing the  temperatures  at  which  freezing  begins  and  at 
which  it  ends  in  each,  the  liquidus  and  solidus  lines  can 
be  located. 

In  the  manganese-copper  diagram,  Fig.  21,  the  rela- 
tionships along  the  line  xx'  are  wholly  similar  to  those 
just  described  except  that  the  primary  separation  is  a 
copper-rich  crystal.  The  alloy  having  the  composition 
y  (copper  70  per  cent.-manganese  30  per  cent.)  is 
unique  in  that  it  is  the  only  one  of  the  series  in  which 
the  composition  of  the  crystal  first  separating  is  the  same 
as  the  liquid  melt  from  which  it  comes.  The  freezing 
which  takes  place  at  M  is,  therefore,  analogous  to  the 
freezing  of  a  pure  metal  and  its  cooling  curve  is  repre- 
sented by  two  sloping  lines  connected  by  a  horizontal. 

Microscopic  Appearance. — The  microscopic  appear- 
ance of  solid  solutions  is  often  misleading  but  is  readily 
explained.  If  perfect  equilibrium  conditions  are  reached, 
the  solid  solution  should,  and  does,  resemble  a  pure 
metal.  Consideration  of  the  method  of  freezing  makes 
it  clear  that,  unless  time  is  allowed  for  the  readjustment 
of  the  varying  crystal  concentrations  during  freezing,  the 
crystal  has  a  duplex  rather  than  a  homogeneous  struc- 
ture. In  the  case  of  manganese-copper,  rapid  cooling 
of  a  copper-rich  mixture  produces  an  alloy  showing  leaf- 
like  masses  of  a  copper-rich  alloy  imbedded  in  a  ground- 
mass  of  manganese  alloy.  A  true  solid  solution  can 
always  be  made  homogeneous  by  annealing,  i.e.,  heating 
.for  a  long  period  at  some  temperature  below  the  melting 
point  of  the  crystal,  so  that  opportunity  is  given  for  the 
concentrations  of  the  duplex  crystals  to  equalize  each 
other  by  diffusion. 

A  limited  use  has  been  found  for  copper-manganese 


48 


PRINCIPLES  OF  METALLOGRAPHY 


alloys  of  high  copper  concentration  in  firebox  stay  bolts. 
An  alloy  containing  82  per  cent,  copper,  15  per  cent, 
manganese  and  the  rest  nickel  and  iron  is  the  interesting 
alloy,  Manganin,  which  has  a  very  high  electrical  re- 
sistance and  a  temperature  coefficient  of  almost  zero. 

A  second  type  of  solid  solution  diagram  which  might 
be  expected  would  be  one  in  which  the  addition  of  each 
metal  to  the  other  raised  both  melting  points  producing  a 
solid  solution  curve  with  a  maximum.  This  type  has 
not  been  found  in  practice. 

A  third  and  very  important  type  of  solid  solution 
includes  that  class  of  alloys  which  are  mutually  soluble 


9|0         8|0         7|0         C|0        5|0         4|0         3|0        2|0 


FIG.  23.  —  Copper-nickel  alloys  (GUEKTLER  AND  TAMMAN)  . 

in  all  proportions  but  whose  diagrams  show  neither  a 
maximum  nor  a  minimum.  A  relation  of  this  sort  is 
shown  in  the  copper-nickel  alloys,  Fig.  23.  The  mech- 
anism of  cooling  is  exactly  the  same  as  in  the  case  of  the 
copper-manganese  alloys,  except  that  there  is  no  alloy 
in-  the  series  which  on  crystallizing  has  the  same  com- 
position as  the  liquid  from  which  it  comes.  These 
copper-nickel  alloys  are  of  great  technical  importance. 
Some  compositions  are  used  for  copper  coins,  an  alloy 
containing  20  per  cent,  nickel  is  used  for  capping  rifle 
bullets  and  the  alloy  60  per  cent,  copper  and  40  per 
cent,  nickel  has  high  electrical  resistance  and  a  low 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING  49 

temperature  coefficient.  Under  the  name  Constantan 
this  latter  is  often  used  for  electrical  purposes  and  coupled 
with  copper  or  iron  wire  forms  an  excellent  thermo- 
couple as  was  stated  in  the  previous  chapter  (p.  24). 
One  of  the  important  Canadian  ores  is  of  such  a  composi- 
tion that,  on  smelting,  there  is  produced  a  copper-nickel 
alloy  containing  about  67  per  cent,  nickel,  28  per  cent, 
copper  and  a  small  amount  of  iron  and  manganese. 
The  alloy  is  known  as  Monel  metal  and  has  very  great 
tensile  strength,  high  ductility  and  remarkable  resistance 
to  corrosion. 

That  group  of  alloys  in  which  the  constituents  show 
more  or  less  complete  solubility  in  each  other  includes 
by  far  the  greater  number  of  the  technically  important 
alloys.  Steel,  brass  and  the  bronzes  all  belong  in  this 
class  and  will  be  discussed  in  detail  in  later  chapters. 
Brief  reference  must  be  made  here  to  a  few  of  the  less 
commonly  used  alloys  of  this  class  which  are,  neverthe- 
less, very  important  technically.  Gold,  with  dissolved 
silver,  is  the  basis  of  gold  coinage  and  jewelry.  White 
gold  used  to  imitate  platinum  is  an  alloy  of  gold  with 
about  18  per  cent,  of  nickel.  Of  interest  to  the  chemist 
are  Palau,  the  palladium-gold  alloy  used  as  a  substitute 
for  platinum,  nichrome  or  chromel  the  highly  resistant 
nickel-chromium  solid  solution  much  used  in  triangles 
for  laboratory  use,  for  crucible  tongs  and  in  heating 
coils  for  electric  furnace  work,  and  ste  lite  the  cobalt- 
chromium  alloy  which  because  of  its  remarkable  non- 
rusting  properties  finds  varied  uses  in  the  manufacture 
of  cutlery,  surgical  instruments  and  the  like. 

Intermetallic  Compounds.1 — A  large  number  of  alloys 
contain  or  consist  of  definite  intermetallic  compounds 
of  which  more  than  three  hundred  have  been  found. 
With  few  exceptions,  these  alloys  are  technically  unim- 

1  DESCH,  " IntermetWlic  Compounds." 


50  PRINCIPLES  OF  METALLOGRAPHY 

portant  as  they  are  for  the  most  part  hard,  brittle  and 
almost  wholly  lacking  in  strength  and  ductility.  They 
are  interesting  chiefly  from  the  viewpoint  of  the  stu- 
dent of  valence  as  many  of  the  compounds,  while  rela- 
tively simple,  show  valence  relations  differing  greatly 
from  those  that  have  been  generally  accepted,  as  for 
example,  Na  Znn. 

The  effect  of  the  existence  of  a  compound  on  the  shape 
of  the  equilibrium  curve  depends  on  whether  the  com- 
pound does  or  does  not  decompose  into  its  elements 
before  its  melting  point  is  reached.  If  the  compound 
does  not  decompose  before  it  melts,  the  diagram  is  of 
the  open  maximum  type.  If  it  does  decompose  before 
the  melting  point  is  reached,  the  diagram  is  said  to 
show  a  concealed  maximum  or  to  be  of  the  transition 
type. 

The  Open  Maximum. — The  alloys  of  tin  and  magnesium 
illustrate  the  first  type.  Assume  for  the  purposes  of 
discussion,  that  magnesium  and  tin  do  unite  to  form  the 
compound  SnMg2  and  that  this  compound,  which  is 
homogeneous  and  behaves  in  every  way  like  a  pure 
metal,  is  completely  miscible  both  with  magnesium 
and  with  tin  in  the  molten  state  but  wholly  insoluble, 
in  both,  in  the  solid  state.  In  such  a  case  we  would  be 
dealing  with  two  systems  of  the  simple  eutectic  type 
as  shown  in  Fig.  24,  A  and  B. 

If  these  two  simple  diagrams  are  combined  as  in  Fig. 
24,  C  the  result  is  a  typical  compound  diagram  showing  a 
maximum  at  a  point  corresponding  to  the  compound 
SnMg2.  In .  laboratory  practice  there  are  four  indica- 
tions of  the  existence  and  composition  of  a  compound 
of  this  type.  Representing  the  two  metals  as  A  and  B 
and  the  compound  as  AmBn  and  assuming  that  AmBn 
does  not  dissolve  either  A  or  B  to  form  a  solid  solution, 
it  will  be  seen  that:  $ 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING 


51 


1.  The  compound  AmBn  lies  at  the  maximum  of  the 
curve. 

2.  The  eutectic  temperature-hold  corresponding  to  a 
eutectic  between  AmBn  and  A  becomes  zero  at  A  and  at 
the  composition  AmBn. 

3.  The  eutectic  between  AmBn  and  B  also  disappears 
at  AmBn. 


FIG.  24. — Development  of  open  maximum  type  diagram  (Tin-magnesium 
alloys — GBUBB). 

4.  The  alloy  corresponding  to  AmBn  is  the  only  one 
which  is  microscopically  homogeneous. 
-  Unfortunately  the  methods  of  locating  the  compound 
are  seldom  as  exact  as  in  this  case.  Far  more  often  the 
compound  will  dissolve  one  or  both  of  the  metals  from 
which  it  is  made,  to  form  solid  solutions,  as  indicated  in 
Fig.  25.  Under  these  circumstances  criteria  2,  3,  and  4 
disappear  as  neither  eutectic  E!  nor  eutectic  E2  ends 
at  the  composition  AmBn  and  (4)  the  alloy  is  homo- 
geneous not  only  at  AmBn  but  throughout  the  entire 
range  between  D  and  F.  In  such  a  case  the  intersection 


52 


PRINCIPLES  OF  METALLOGRAPHY 


of  the  maximum  curve  with  its  tangent,  drawn  parallel 
to  the  concentration  axis,  indicates  the  composition 
of  the  compound.  This  point  will  invariably  be  found 
to  lie  at,  or  very  near,  a  composition  corresponding  to  a 
simple  atomic  relationship. 


FIG.  25. — Open  maximum  type  with  solid  solutions. 

The  Concealed  Maximum. — This  is  perhaps  the  most 
difficult  of  all  alloy  types  to  understand  and  because  of 
experimental  difficulties,  to  be  referred  to  shortly,  is  the 
hardest  to  construct.  An  ideal  case  is  illustrated  in 
Fig.  26. 

The  elements  A  and  B  unite  to  form  a  compound 
AmBn  but,  instead  of  melting  as  a  homogeneous  sub- 
stance the  compound  decomposes  into  its  elements  at  a 
temperature,  CD,  below  its  melting  point.  The  result  is 
that,  instead  of  passing  directly  from  a  solid  compound 
into  a  molten  metal,  it  changes  at  the  temperature  CD, 
into  a  liquid  of  composition  D  and  a  solid  of  different  com- 
position from  the  original  solid,  namely,  the  metal  A. 
Conversely,  when  a  mixture  of  the  composition  AmBn 
is  cooled  from  a  liquid  the  following  changes  take  place. 
The  temperature  falls  normally  until  the  point  /  on 
the  line  AD  is  reached.  Here,  as  the  diagram  shows, 
pure  A  begins  to  separate  and  the  concentration  changes 
in  the  usual  way  until  the  point  D  is  reached.  The 
temperature  has  now  fallen  to  that  point  at  which  the 
compound  AmBn  can  exist  and  the  tendency  for  it  to 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING 


53 


form  is  so  great  that  a  reaction  takes  place  between  the 
solid  crystals  of  A  which  are  in  suspension  and  the 
liquid  of  the  composition  D.  Since  the  crystals  A  are 
obviously  far  poorer  in  B  than  is  the  compound  and  since 
the  liquid  D  is  much  richer  in  B  than  the  compound,  a 
reaction  between  the  two  to  form  the  compound  causes 
the  complete  disappearance  of  both.  If  the  original 


ULA    9|0         8|0 HO 610 510 410 310 210 1UL 

,-  ,  Composition 

FIG.  26.    ' 

mixture  has  a  composition,  F,  richer  in  metal  A  than  the 
compound,  not  all  of  the  A  crystals  can  be  used  up  by  the 
liquid  D  and  the  solid  alloy  is  a  mixture  of  A  and  AmBn. 
If,  on  the  other  hand,  the  original  mixture  has  the  com- 
position G,  containing  less  A  than  corresponds  to  the 
compound,  it  follows  that  the  A  crystals  which  separate 
first  along  the  line  CD  will  be  wholly  dissolved  by  the 
liquid  D  and  a  certain  amount  of  excess  liquid  will  be 


54  PRINCIPLES  OF  METALLOGRAPHY 

left.  Crystallization  then  proceeds  along  the  line  DE 
until  the  eutectic  (a  mixture  of  AmBn  and  B)  is  reached. 

Since  D  represents  that  point  at  which  the  pure  metal 
A  reacts  with  the  liquid  and  is  transformed  into  the 
compound  AmBn,  D  is  often  called  a  transition  point  and 
AmBn  a  transition  product. 

As  was  the  case  with  the  open  maximum,  there  is  no 
difficulty  in  locating  the  compound  in  the  ideal  type  of 
concealed  maximum  just  considered,  because  the  tem- 
perature change  corresponding  to  the  eutectic  E  is  zero 
at  the  composition  of  the  compound,  and,  as  in  the  first 
case,  the  compound  is  the  only  homogeneous  alloy  in  the 
series.  The  time  during  which  the  temperature  stays 
constant,  while  the  transformation  on  the  line  CD  is 
taking  place,  is  longest  under  ideal  conditions  at  the  com- 
position of  the  compound,  dropping  to  zero  at  points  C 
andD. 

Solid  solutions  complicate  the  concealed  maximum 
diagram  as  they  do  the  open  maximum  diagram  but  there 
is  the  additional  serious  difficulty  of  incomplete!  trans- 
formation at  the  point  D.  This  is  readily  understood 
from  the  nature  of  the  reaction  taking  place  at  thi^  point. 
The  pure  crystals  of  A  begin  to  dissolve  in  liquid  D  to 
form  the  compound.  This  formation  takes  place  on  the 
surface  of  the  crystal  and  not  infrequently  the  compound 
forms  a  coating  around  the  crystal  protecting  it  from 
further  reaction  with  the  solution  D  as  indicated  dia- 
grammatically  in  the  sketch,  Fig.  27,  and  actually  in  the 
photomicrograph,  Fig.  28.  . «' 

This  phenomenon  is  known  as  surrounding  and  makes 
the  location  of  the  compound  exceedingly,  difficult  for 
several  reasons.  First,  the  time  during  which  the  tem- 
perature remains  constant  at  the  transition  point,  which 
normally  is  longest  at  the  composition  of  the  compound, 
is  greatly  reduced  if  the  transformation  is  incomplete 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING 


55 


and  instead  of  decreasing  regularly  to  the  left  and  right 
of  the  compound,  changes  so  irregularly  that  the  time 
curve  is  of  no  value.  Second,  since  more  of  the  liquid 
is  left  than  corresponds  to  true  equilibrium,  the  eutectic 
line  HER,  Fig.  26,  extends  to  the  left  of  its  normal  limit 
in  some  such  way  as  in  indicated  by  the  dotted  line  KK\. 
Finally,  the  alloy,  even  though  it  corresponds  to  the 
compound,  may  show  three  structure  elements,  Fig.  27. 
In  a  case  of  this  sort  practically  all  that  can  be  done  to 


FIG.  27. — Sketch  illustrat- 
ing the  phenomenon  of  sur- 
rounding. 


FIG.  28. — Antimony  50  per 
cent.  Tin  50  per  cent.  Shows 
SbSn  inclosing  crystals  of 
antimony.  The  black  ground 
mass  is  a  tin-rich  solid  solu- 
tion. 


locate  the  compound  is  to  anneal  a  series  of  alloys  in  the 
neighborhood  of  the  compound  and  determine  which 
one  of  the  series  becomes  homogeneous.  Should  several 
alloys  in  the  series  become  homogeneous,  indicating  the 
existence  of  solid  solutions,  the  determination  of  the 
composition  of  the  compound  is  practically  impossible. 

The  hardness  of  the  intermetallic  compounds  is  their 
most  useful  property  and  is  made  use  of  in  the  few 
technical  alloys  in  which  they  are  found.  In  bearing 
metals,  for  example,  where  a  small  amount  of  hard 
material  not  easily  worn  down  by  abrasion  is  desired,  the 
compound  Cu3P  is  found  in  phosphor  bronze  and  the 


56  PRINCIPLES  OF  METALLOGRAPHY 

compounds  SbSn  and  Cu3Sn  in  Babbitt  metal.  By 
far  the  most  important  of  these  compounds  is  the  iron- 
carbon  compound  Fe3C,  the  chief  surface  constituent  of 
case  hardened  steel,  the  preparation  of  which  will  be  con- 
sidered later. 

Changes  in  the  Solid  Alloy. — The  diagrams  which  have 
been  considered  so  far  have  dealt  with  changes  which 
occur  when  the  alloy  passes  from  the  liquid  to  the  solid 
state  or  vice  versa.  Some  of  the  most  valuable  technical 
alloys,  notably  steel,  acquire  their  properties  or  modify 
them  materially  by  changes  which  take  place  in  the  solid 
state.  Iron,  for  example,  is  believed  to  exist  in  at  least 
"three  allotropic  forms,  a-iron  stable  below  780°,  /3-iron 
existing  between  780°  and  900°  and,  finally,  7-iron  stable 
above  900°  and  practically  non-magnetic.  While  these 
magnetic  changes  are  of  interest  to  the  physicist,  the 
fact  of  importance  to  the  metallographist  is  that  7-iron 
will  hold  carbon  in  solid  solution  and  that  a-iron  will  not. 
This  means  that,  when  the  iron-carbon  alloy  is  cooled, 
a  change  in  components,  and  therefore  in  physical  prop- 
erties, occurs  in  passing  from  the  7-iron  to  the  a-iron 
range  even  though  the  alloy  in  the  7-field  is  perfectly 
solid. 

More  important  than  changes  due  to  the  allotropism 
of  a  single  metal  are  those  changes  which  come  from  the 
decomposition  of  a  solid  solution  at  temperatures  below 
its  freezing  point.  All  the  changes  which  can  take  place 
when  a  liquid  solution  freezes  may  also  occur  when  a 
solid  solution  decomposes.  It  may  change  to  a  eutectic- 
like  mixture  or  to  another  solid  solution  more  or  less 
complete,  or  it  may  decompose  to  form  one  or  more  com- 
pounds. Some  of  these  possible  changes  are  indicated  in 
the  sketch,  Fig.  29.  The  most  important  of  these  trans- 
formations in  the  solid  state  is  that  shown  in  A  of  this 
sketch  as  many  of  the  valuable  properties  of  steel,  given 


THE  ALLOY  DIAGRAM  AND  ITS  'MEANING 


57 


to  it  by  heat  treatment,  are  due  to  a  decomposition  of 
this  sort.  Although  the  details  will  be  given  later  it 
may  be  said  that  the  transformation  from  a  solid  alloy 
of  the  solid  solution  type  to  one  of  a  different  character 
requires  a  definite  amount  of  time  and,  by  shortening 
the  time,  this  transformation  can  be  partially  or  wholly 
suspended.  For  example,  by  suddenly  cooling  (quench- 
ing) an  alloy  from  the  temperature  indicated  by  x  in 
Fig.  29,7,  it  is  possible  to  prevent  the  transformation 
along  AE  of  the  solid  alloy  into  that  represented  by  x' 
with  the  result  that  at  ordinary  temperature  the  alloy 


in 


FIG.  29. — Types  of  changes  occurring  in  the  solid  state. 

exists  in  the  condition  which  it  had  at  the  higher  tem- 
perature x.  The  physical  properties  of  a  solid  solution 
are  so  different  from  those  of  a  eutectic-like  mixture  that 
by  more  or  less  completely  checking  the  change  from  x 
to  x1  the  mechanical  properties  of  the  alloy  can  be  pro- 
foundly modified  and  can  be  controlled  within  fairly 
definite  limits.  The  alloy  represented  by  the  point  E, 
Fig.  29,7,  has  all  the  characteristics  of  the  eutectic  mix- 
ture which  separates  from  a  liquid  solution.  Since  the 
separation  takes  place  from  a  solid  solution,  however, 
the  name  eutectoid  is  commonly  given  to  it. 

Many  binary  diagrams  of  a  much  more  complex  char- 
acter might  be  discussed  but  all  of  them  are  combina- 


58 


PRINCIPLES  OF  METALLOGRAPHY 


tions  of  the  simpler  diagrams  and  can  be  constructed  or 
interpreted  without  difficulty.  It  is  only  necessary  to 
break  down  the  complex  diagrams  into  the  simpler 
elements  of  which  they  are  composed  in  order  to  make 
them  perfectly  clear.  As  a  single  example  of  a  combina- 
tion diagram  that  of  the  copper-antimony  series  of  alloys 
may  be  noted,  Fig.  30.  In  this  is  found  the  open  maxi- 
mum, probably  SbCu3,  the  concealed  maximum  SbCu2, 


Composition 
FIG.  30. 

a  eutectic  mixture  of  SbCu2  and  Sb  and  a  series  of  solid 
solutions,  a.  Examination  of  this  fairly  complex  dia- 
gram shows  clearly  what  combinations  are  to  be  expected 
at  different  temperatures  and  at  varying  compositions. 

Ternary  Alloys. — The  alloys  of  three  or  more  metals 
form  a  large  and,  to  a  great  extent,  unworked  field  of 
metallography.  The  experimental  work,  while  no  more 
difficult  than  for  the  binary  mixtures,  is  much  more 
expensive  and  time  consuming  because  of  the  large 
number  of  experiments  needed  to  establish  the  relation- 
ships. The  study  of  ternary  systems  will  be  extended 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING 


59 


in  time  and  will,  doubtless,  lead  to  the  discovery  of 
interesting  and  commercially  important  alloys. 

Anything  like  a  complete  discussion  of  ternary  dia- 
grams is  beyond  the  scope  of  this  book  but  a  single 
example  will  be  considered  to  show  the  methods  of  rep- 
resenting these  alloys  and  the  way  in  which  the  diagrams 
are  constructed  from  experimental  data. 

It  is  evident  that  the  relation  of  three  metals  cannot  be 
represented  in  a  plane  as  in  the  case  of  the  binary  alloys. 
The  easiest  way  to  visualize  these  relationships  is,  per- 
haps, in  the  form  of  a  space  model,  Fig.  31,  on  a  triangular 


Fio.  31. — The  ternary  solid   (the  intersection  of  the  dotted  lines  Et  is  the 
ternary  eutectic). 

base,  the  corners  of  which  represent  the  three  component 
metals,  while  the  perpendiculars  to  this  triangular  plane 
represent  the  temperatures.  Such  a  method  of  represen- 
tation has  many  disadvantages.  It  is  difficult  to  construct 
mechanically  and  unsatisfactory  in  that  it  shows  only 
the  liquidus  surface,  i.e.,  the  surface  on  which  solids 
begin  to  form,  and  gives  no  idea  of  the  changes  which 
take  place  beneath  the  surface.  A  more  satisfactory 
method  consists  in  the  projection  on  a  triangular  surface 
of  a  series  of  contour  lines  indicating  temperature 


60  PRINCIPLES  OF  METALLOGRAPHY 

changes.  Such  a  figure,  together  with  a  series  of  tri- 
angular diagrams  showing  the  conditions  which  exist  at 
varying  stages  of  the  crystallization  will  usually  be 
enough  to  indicate  the  various  products  formed.1 

In  the  graphic  representation  of  a  triple  alloy  the  first 
step  is  a  means  of  showing  the  composition  of  any  given 


7WVV\ 
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.  AAAAAA/^oi%\AAAAAA7 , 

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i    AAA    A 

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%Bi.lQ         20          y          40          50          60          70          80  Z     £ 
FIG.  32. — Method  of  showing  the  composition  of  ternary  alloys. 

mixture.  This  is  done  most  easily  by  the  use  of  plotting 
paper  with  triangular  coordinates.  The  classic  example 
of  the  simple  ternary  is  the  series  composed  of  lead,  tin 
and  bismuth.  In  the  diagram,  Fig.  32,  the  three  corners 
represent  the  pure  metals.  The  base  of  the  triangle 
of  which  each  metal  is  the  apex  represents  therefore  zero 
concentration  of  that  element.  For  example,  the  line 

1  An  excellent  discussion  of  Ternary  Alloys  will  be  found  in  GULLIVER, 
"Metallic  Alloys,"  Ed.  2,  p.  340,  from  which  much  of  the  following 
material  is  taken. 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING 


01 


Pb-Sn  represents  zero  per  cent.  Bi,  the  line  Pb-Bi  zero 
per  cent.  Sn  and  Bi-Sn  is  zero  per  cent.  Pb.  Starting  with 
these  lines  as  bases  and  approaching  the  element  whose 
percentage  is  desired,  each  line  parallel  to  the  base 
indicates  one  per  cent,  assuming,  as  is  usually  the  case, 
that  the  triangle  is  divided  by  100  parallel  lines  in  each 
of  the  three  directions.  Point  X  in  the  figure  there- 
fore represents  20  per  cent.  Pb,  50  per  cent.  Sn  and 
30  per  cent.  Bi. 

A  study  of  the  geometry  of  this  figure  shows  two  other 
facts  which  are  of  importance  in  the  actual  construction 


FIG.  33. — Combination  of  the   binary  surfaces — Pb-Sn,  Sn-Bi  and  Bi-Pb. 

of  these  diagrams.  A  line  drawn  from  any  corner  to 
any  point  on  the  opposite  side  represents  an  alloy  in 
which  two  of  the  metals  have  a  constant  relationship  to 
each  other  while  the  percentage  of  the  third  metal  varies. 
Pb-Y,  for  example,  is  a  line  on  which  the  relation  of  tin  to 
bismuth  is  always  7  to  3.  A  line  parallel  to  one  side  of 
the  triangle  represents  an  alloy  in  which  one  metal  has  a 
constant  percentage  while  the  other  two  vary.  On 
WZ  the  per  cent,  of  Sn  is  always  15  per  cent,  while  the  lead 
increases  from  zero  per  cent,  at  Z  and  the  bismuth  from 
zero  per  cent,  at  W.  \  • 
The  next  step  in  the  construction  of  the  model  would 


62 


PRINCIPLES  OF  METALLOGRAPHY 


be  to  erect  three  plane  figures  on  the  three  edges  of  the 
triangle  corresponding  to  the  three  binary  alloys.  The 
sketch  (Fig.  33)  shows  the  appearance  of  such  a  space 
model. 

If  lead  is  then  added  to  the  binary  eutectic  of  lead  and 
tin  the  melting  point  of  the  mixture  is  lowered.  The  same 
effect  is  noticed  when  tin  is  added  to  the  lead-tin  eutectic 
and  when  bismuth  is  added  to  lead-tin.  Since  these  three 


?t   250  225°  200   175   150° 


FIG.  34. — Ternary  diagram  with  contour  lines  (CHAHPT). 

lines  all  slope  downward  their  intersection  must  lie  at  a 
point  lower  than  the  melting  point  of  any  of  the  binary 
eutectics.  This  point  is  shown  at  E,  Fig.  31  (p.  59)  and 
is  the  ternary  eutectic.  The  shape  of  the  space  model  for 
an  alloy  of  this  type  will  be  apparent  from  the  consider- 
ations j ust  outlined.  The  three  lines  connecting  the  single 
ternary  eutectic  with  the  three  binaries  form  the  lower 
edges  of  the  three  valleys  made  by  the  intersections  of 
the  three  curved  surfaces,  composing  the  liquidus  surface. 
If  this  surface  is  projected  on  its  triangular  base  and 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING  63 

contour  lines  drawn,  representing  the  intersections  of  a 
series  of  parallel  horizontal  planes  with  the  space  model 
a  figure  of  the  shape  shown  in  Fig.  34  results.  These 
contour  lines,  which  are  naturally  isothermals  (lines  of 
constant  temperature),  show  that  the  surfaces  above  the 
binary  eutectic  valleys  are  convex. 

Consider,  next,  the  changes  which  take  place  on  cooling 
such  an  alloy  as  is  represented  by  X  in  the  diagram.  As 
the  temperature  falls,  pure  lead  separates  along  the  line  X- 
Xi,  the  relationship  of  bismuth  and  tin  staying  constant. 
At  Xi  enough  lead  has  separated  so  that  lead  and  tin  are 
in  the  eutectic  ratio  and  the  two  metals  crystallize  to- 
gether along  the  bottom  of  the  binary  eutectic  valley 
until  the  composition  E  is  reached,  at  which  point  the  re- 
maining liquid  solidifies  as  the  ternary  eutectic .  Although , 
accurately  defined,  a  eutectic  should  be  of  constant  melt- 
ing point,  the  binary  mixtures  of  eutectic  composition 
vary  in  melting  point  with  the  amount  of  the  third 
element.  As  a  matter  of  easy  statement,  however,  the 
mixtures  of  binary  eutectic  composition  are  universally 
referred  to  as  binary  eutectics.  If  a  number  of  ternary 
alloys  of  Pb,  Sn  and  Bi  are  studied  and  the  points  deter- 
mined at  which  the  binary  eutectics  begin  to  form,  planes 
drawn  through  these  points  will  give  the  binary  eutectic 
surface.  This  is  found  to  consist  of  six  twisted  surfaces, 
each  intersecting  its  neighbor  in  such  a  way  that  there 
will  be  three  ridges,  the  binary  eutectic  lines,  and  three 
valleys,  the  projections  of  which  connect  the  ternary 
point  E  with  the  three  corners  of  the  triangle.  Since  all 
alloys  of  the  series  become  solid  at  the  temperature  of  the 
ternary  eutectic,  the  solidus  surface  is  a  horizontal  plane 
through  the  ternary  eutectic  point  E. 

In  the  actual  construction  of  these  ternary  diagrams, 
the  common  practice  is  to  study  a  number  of  vertical 
sections  from  which  the  space  model  or  its  projection  can 


64  PRINCIPLES  OF  METALLOGRAPHY 

be  assembled.  In  the  lead-tin-bismuth  series,  for  example, 
a  fairly  complete  study  of  the  alloys  of  the  compositions 
represented  by  the  lines  Pb-A,  Sn-B  and  Bi-C,  Fig.  34, 
gives  a  general  idea  of  the  shape  of  the  model,  and  the 
necessity  for  further  study  in  the  area  Bi-O-B  is  apparent. 
Referring  again  to  Fig.  32,  it  will  be  seen  that  a  series 
of  alloys  starting  with  the  composition  represented  by  X 
and  with  gradually  increasing  percentages  of  bismuth  (sec- 
tion X-Bi)  will  give  much  additional  information  with  re- 
gard to  conditions  in  the  eutectic  area.  This  series  might 
be  followed  by  another  section  in  which  the  percentage  of 
bismuth  is  kept  constant  at  50  per  cent.,  while  lead  and 
tin  are  varied.  By  studying  several  sections  in  this  way 
it  is  soon  possible  to  construct  the  space  model  accurately. 

Microscopic  Appearance  of  Ternary  Alloys. — The 
microscopic  study  of  these  alloys  is  not  satisfactory. 
The  primary  crystals  are  perfectly  normal  but  the  binary 
eutectic  separations  occur  so  slowly  and  over  so  consid- 
erable a  temperature  range  that  segregation  generally 
takes  place  and  the  normal  eutectic  structure  is  lost. 
The  ternary  eutectic  is,  also,  so  finely  divided  and  so 
intimate  a  mixture  that  the  component  elements  can  be 
found  only  by  careful  double  etching  and  then  with 
much  difficulty.  That  three  distinct  structure  elements 
are  present,  however,  is  shown  in  the  photograph,  Fig.  35. 

In  the  class  of  technically  important  ternary  alloys 
are  included  Babbitt  metal  and  nickel  silver  (formerly 
called  German  silver).  The  composition  of  Babbitt 
metal  varies  over  a  considerable  range  but  it  is  usually 
an  alloy  of  tin,  antimony  and  copper.  A  common  com- 
position is  tin,  90  per  cent.,  antimony,  7  per  cent, 
and  copper,  3  per  cent.  Microscopic  examination 
shows  the  alloy  to  consist  of  crystals  of  SbSn  and 
Cu3Sn  imbedded  in  a  tin  matrix.  Babbitt  metal  is 
an  antifriction  alloy  and  because  of  its  comparatively  low 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING  65 

tensile  strength  is  commonly  used  to  line  bronze  bushings, 
the  bronze  giving  the  needed  strength  and  the  Babbitt 
the  low  frictional  resistance.  The  microscope  is  of  great 
use  in  the  making  of  Babbitt  lined  bearings  as  by  its 
aid  it  is  possible  to  detect  segregation  in  the  metal  and 
also  to  determine  the  size  of  the  SbSn  crystals.  Large 
crystals,  due  to  very  slow  cooling  when  the  Babbitt  is 
cast,  and  excessively  fine  crystals,  due  to  a  sudden  chilling 
of  the  molten  metal,  both  produce  unsatisfactory  bearing 


FIG.  35. — Ternary  diagram  of  lead,  tin  and  bismuth.     75  X  (HOMEHBERG). 

surfaces.  A  temperature  of  approximately  100°C.  for 
the  mould  has  been  found  to  give  satisfactory  results. 

Nickel  silver  is  composed  of  copper,  nickel  and  zinc. 
It  has  a  wide  use  in  the  production  of  non-corrodible 
articles,  table  ware  (with  or  without  silver  plating) ,  and 
the  like. 

Alloys  with  four  or  more  constituents  are  not  un- 
common. Some  of  them  find  important  application  in 
the  manufacture  of  easily  melted  fuse  plugs  for  automatic 
sprinkler  systems.  Wood's  metal  corresponds  to  the 


66  PRINCIPLES  OF  METALLOGRAPHY 

ternary  eutectic  of  Bi,  Pb  and  Sn  to  which  a  small 
amount  of  cadmium  is  added.  It  melts  at  70°C. 

The  Phase  Rule.1 — This  chapter  on  equilibrium 
diagrams  would  not  be  complete  without  reference  to  the 
Phase  Rule  which,  while  it  is  of  little  use  to  the  technical 
metallographist,  has  been  a  most  valuable  tool  in  the 
study  of  equilibrium  diagrams,  making  it  possible  to  state 
in  any  given  case  what  the  equilibrium  conditions 
actually  are.  Knowing  the  conditions  which  should 
exist  at  equilibrium,  the  microscope  makes  it  possible 
to  decide  at  once  whether  or  not  equilibrium  has  been 
reached. 

The  general  statement  of  the  phase  rule  is  as  follows : 

F  =  C  +  2 -P 

in  which  F  represents  the  number  of  degrees  of  freedom' 
C  the  number  of  components  and  P  the  number  of  phases. 
In  the  general  case  these  factors  are  often  difficult  to 
define  but  in  the  application  to  metallic  alloys  no  such 
difficulty  is  met.  The  components,  C,  are  obviously  the 
metals.  The  degrees  of  freedom  are  the  changes  which  the 
alloy  can  undergo,  namely,  changes  of  temperature,  con- 
centration and  pressure.  Since  vapor  can  be  neglected 
with  most  alloys  and  since  the  pressures  commonly  met 
in  alloy  practice  are  too  small  to  have  any  appreciable 
effect,  changes  in  pressure  can  be  omitted,  reducing  the 
variables  (degrees  of  freedom)  to  temperature  and  con- 
centration. A  phase  is  denned  as  a  homogeneous, 
physically  distinct  substance.  In  dealing  with  alloys,  it 
may  be  a  pure  metal,  a  metallic  compound  or  a  solid 
solution.  In  addition,  each  physical  state  of  the  sub- 
stance, whether  solid,  liquid  or  gas  constitutes  a  separate 
phase. 

Neglecting  the  vapor  phase  and  the  effect  of  pressure, 

1  FINDLEY,  "The  Phase  Rule  and  Its  Applications." 


THE  ALLOY  DIAGRAM  AND  ITS  MEANING 


67 


the  Phase  Rule  for  alloys  may  be  reduced  to  the  simple 
form 

F  =  C  + 1  -P 

The  number  of  components  in  a  binary  alloy  is  2,  so  the 
expression  is  still  further  simplified  and  takes  the  form 

F  =  3  -P 

A  concrete  illustration  of  the  use  of  the  Phase  Rule  is 
given  in  the  following  diagram,  Fig.  36.     A  point  at 


r 


:        Divariant 
(Temp,  and  cone, 
may  change ) 


Monov'ariant 
(Temp,  or  cone) 
may  change  but 

not  both) 


Non-variant 
.(Neither  temp. 

nor.  cone. 
may  change) 


I  Triple  point 
(Liquid  and  two 
Solids  may  exist  here) 


Concentration 
FIG.  36. — Phase  rule  diagram  for  binary  alloys. 

X  lies  in  the  liquid  phase.  Substituting  1  in  the  sim- 
plified expression  it  becomes,  F  =  3  —  1  =  2.  In  words, 
the  alloy  now  possesses  two  degrees  of  freedom.  Both 
temperature  and  composition  can  be  varied  within  the 
area  bounded  by  AEB  and  the  alloy  will  stay  molten. 
This  field  then  represents  an  area  of  divariant  equilibrium. 
At  the  point  Xi  on  the  line  AE,  the  crystal  is  beginning 
to  separate  but  is  in  contact  with  the  liquid.  Two  phases, 
crystal  and  liquid,  are  present  and  the  expression  becomes, 


68  PRINCIPLES  OF  METALLOGRAPHY 

F  =  3  —  2  =  1.  The  alloy,  now,  has  only  one  degree 
of  freedom,  for  any  change  in  temperature  is  accompanied 
by  a  change  in  concentration  along  the  line  AE,  and  a 
change  in  concentration  necessitates  a  change  in  tempera- 
ture. The  lines  AE  and  BE  are  therefore  lines  of  mon- 
ovariant  equilibrium.  At  the  point  E,  the  eutectic 
point,  or  at  any  other  point  on  the  eutectic  line,  CED,  two 
crystal  phases,  A  and  B,  are  in  contact  with  a  liquid 
phase  of  composition  E.  (Solid  E  contains  A  and  B 
in  the  form  of  very  fine  crystals.)  Under  these  con- 
ditions, the  expression  becomes  F  =  3  —  3  and  the 
system  becomes  non-variant.  Neither  the  temperature 
of  the  mixture  nor  the  composition  of  the  three  phases 
can  change  until  one  of  the  three  phases  has  disappeared. 
The  temperature  cannot  fall  until  all  the  liquid  phase,  E 
has  solidified  nor  can  it  rise  without  the  disappearance 
of  either  A  or  B.  It  must  not  be  understood  that  the 
mixture  can  have  only  the  composition  represented  by  the 
point  E.  It  may  have  any  composition  along  the  line 
CD  but,  in  this  case,  the  composition  of  each  phase 
remains  the  same,  the  difference  in  original  composition 
producing  changes  in  the  relative  amounts  of  the  three. 
The  line  CD,  therefore,  is  a  line  of  non-variant  equilibrium. 
One  of  the  principle  uses  of  the  Phase  Rule  is  to  deter- 
mine whether  or  not  true  equilibrium  has  been  reached. 
It  is  evident  that,  since  an  alloy  with  less  than  zero 
degrees  of  freedom  is  an  impossibility,  there  can  never  be 
more  than  two  crystal  phases  in  contact  with,  or  separat- 
ing from,  a  two  component  liquid  metal.  Therefore,  in 
a  case  like  that  indicated  in  Fig.  26,  p.  53,  and  illustrated  in 
Fig.  28,  p.  55,  the  presence  of  three  phases,  in  the  micro- 
scopic section  of  the  solid  alloy,  is  a  positive  indication  of 
incomplete  equilibrium. 


CHAPTER  IV 

THE  NON-FERROUS  ALLOYS  OF  TECHNICAL 
IMPORTANCE 

No  attempt  will  be  made  to  describe  or  even  to  name 
the  large  and  constantly  increasing  number  of  non- 
ferrous  alloys  used  in  practice.  Certain  of  them  are  of 
such  great  technical  interest  and  importance,  however, 
that  their  properties  must  be  considered.  Many  have 
been  referred  to  in  connection  with  the  equilibrium  dia- 
grams and  others  are  of  such  special  character  that  their 
consideration  is  out  of  place  here.  The  majority  of  the 
important  non-ferrous  alloys  not  yet  discussed  fall  into 
one  of  two  groups;  the  smaller,  containing  aluminum 
and  its  alloys,  the  larger,  the  alloys  of  copper,  particularly 
the  brasses  and  bronzes. 

Aluminum  Alloys. — While  the  commercial  develop- 
ment of  the  alloys  of  aluminum  has  produced  many 
alloys  of  light  specific  gravity  coupled  with  valuable 
mechanical  properties,  it  has  also  very  great  possibilities 
for  investigation,  notably  along  the  lines  of  alloying  the 
aluminum  base  with  two  or  more  other  metals.  Pure 
aluminum,  as  cast,  has  a  tensile  strength  of  about  14,000 
pounds  per  square  inch,  but  by  cold  work,  as  in  wire 
drawing,  this  may  be  increased  to  nearly  50,000  pounds. 
The  commonest  casting  alloy  is  that  with  a  com- 
position of  92  per  cent,  aluminum  and  8  per  cent,  copper 
which  has  a  tensile  strength  of  about  20,000  pounds  but 
is  more  readily  corroded  than  aluminum  itself. 

The  most  valuable  alloy  is  that  known  as  Duralumin, 
which  is  aluminum  containing  from  3.5  per  cent,  to  5.5  per 
cent,  copper,  0.5  per  cent,  to  0.8  per  cent,  manganese  and 


70  PRINCIPLES  OF  METALLOGRAPHY 

about  0.5  per  cent,  magnesium.  It  has  several  remarkable 
properties  and  is  the  only  aluminum  alloy  which  has  been 
successfully  heat  treated.  As  cast,  the  alloy  has  a 
tensile  strength  of  about  35,000  pounds  per  square 
inch  and  an  elongation  of  17  per  cent,  in  two  inches.  If 
it  is  heated  to  400°-500°C.  and  then  quenched,  there 
is  no  great  change  in  the  physical  properties.  If,  how- 
ever, the  quenched  alloy  is  allowed  to  "age"  for  a  few 
days,  the  tensile  strength  will  increase  to  about  58,000 
pounds  and  the  elongation  to  23  per  cent.  By  hard  rolling, 
duralumin  may  have  its  tensile  strength  increased  to  85,- 
000  pounds  per  square  inch.  In  sheets,  tubes  and  similar 
articles  it  breaks  at  about  50,000  pounds.  Weight  for 
weight,  duralumin  is  as  strong  as  the  best  steel  and  for 
the  same  strength  has  greater  rigidity.  Unlike  other 
aluminum  alloys  containing  copper  it  is  markedly  re- 
sistant to  corrosion  and  compares  favorably  with  copper 
under  similar  conditions.  The  properties  of  duralumin 
make  it  a  most  useful  alloy  for  the  construction  of  various 
articles  especially  in  airplane  and  motor  parts.1 

The  preparation  of  aluminum  alloys  for  metallographic 
examination  presents  some  difficulties.  The  preliminary 
polishing  should  always  be  done  on  emery  papers  mois- 
tened with  oil  and  the  final  polishing  cloths  must  never  be 
allowed  to  dry.  Liquid  abrasives  are  absolutely  neces- 
sary for  successful  results.  The  structure  of  the  polished 
alloys  is  developed  by  immersion  in  dilute  hydrofluoric 
acid  followed  by  treatment  with  nitric  acid.  For  detail 
work  a  0.10  per  cent,  solution  of  sodium  hydroxide  in 
50  per  cent,  alcohol  is  recommended. 

Copper  and  Its  Alloys. — Enormous  quantities  of  pure 
copper  are  used  in  the  electrical  industry  because  of  its 

1  An  excellent  discussion  of  aluminum  alloys  is  given  by  MERICA, 
Chem.  and  Met.  Eng.,  19  (1918),  135  and  Bull.  76,  Bureau  of  Standards, 
April,  1919. 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  71 

very  high  conductivity.  Copper  forms  solid  solutions  with 
many  of  the  common  elements,  including  practically 
all  of  those  with  which  it  is  associated  in  its  production 
from  the  ore.  A  fact  of  great  technical  importance  is 
the  remarkable  effect  of  the  existence  of  solid  solutions 
on  the  conductivity  of  an  alloy.  If  the  alloy  is  of  the 
eutectic  type,  the  elements  are  completely  insoluble  in 
each  other  in  the  solid  state  and  the  conductivity  of 
the  solid  alloy  is  practically  the  sum  of  the  conductivities 


FIG.  37. — Relation  between  solid  solution  and  electrical  conductivity.     A. 
is  the  equilibrium  curve  and  B  the  curve  of  conductivity. 

of  the  component  metals.  When  solid  solutions  occur, 
the  conductivity  drops  off  very  sharply,  the  correspond- 
ing curve  taking  the  form  of  a  steep  sided  U  as  in  Fig.  37, 
in  which  A  is  the  equilibrium  curve  and  B  the  curve  of 
conductivity  corresponding  to  it.  The  rapid  decrease  in 
conduction  due  to  slight  addition  of  the  dissolving 
element  makes  evident  the  harmful  effects  of  even  small 
percentages  of  dissolved  impurities  on  the  conductivity 
of  the  copper  and  the  necessity  of  accurate  analysis  if 
the  metal  is  to  be  used  for  electrical  work.  Oxygen 
dissolves  in  copper  to  an  appreciable  extent  and  also 
unites  with  it  to  form  cuprous  oxide,  Cu20.  The  com- 


72  PRINCIPLES  OF  METALLOGRAPHY 

pound  then  reacts  with  the  copper  to  form  a  eutectic 
series  of  alloys  with  a  eutectic  at  3.5  per  cent.  Cu20 
(0.39  per  cent,  oxygen).  Since  the  presence  of  this  eu- 
tectic has  a  harmful  effect  both  on  the  electrical  and 
mechanical  properties  of  copper,  all  high  grade  copper  is 
deoxidized  in  the  process  of  manufacture.  This  may  be 
done  by  the  use  of  phosphorus,  silicon,  boron  and  prob- 
ably, other  readily  oxidized  elements.  Deoxidizing 
by  means  of  silicon  in  the  form  of  copper  silicide  gives 
copper  with  high  conductivity.  Boron  is  highly 
effective  as  a  deoxidizer  and  boronized  copper  is 
not  infrequently  specified  for  electrical  work. 

For  general  industrial  purposes,  copper  is  used  in  the 
form  of  rolled  sheets,  tubes,  bars  and  drawn  wires.  These 
mechanical  operations1  have  such  marked  effects  not 
only  on  the  physical  but  on  the  metallographic  proper- 
ties of  copper  that  they  will  be  considered  in  some  detail. 
Copper  as  cast  has  a  tensile  strength  of  from  17,000  to 
20,000  pounds  per  square  inch  and  its  ductility  is  indi- 
cated by  an  elongation  of  from  40  to  50  per  cent,  in  two 
inches.  It  is  possible  by  simple  mechanical  work,  such 
as  rolling  or  drawing,  to  increase  the  tensile  strength  to 
almost  50,000  pounds  per  square  inch.  This  increase 
in  tensile  strength  is  accompanied  by  a  great  increase  in 
hardness  and  a  marked  decrease  in  elongation.  Hard 
brass  wire  may  have  an  elongation  of  only  1  or  2  per 
cent. 

Several  theories  have  been  proposed  to  account  for 
this  phenomenon  of  hardening  by  means  of  mechanical 
work,  a  phenomenon  which  is  by  no  means  confined  to 
pure  copper  but  is  a  general  property  of  metals  and  alloys. 
The  most  logical  theory  and  the  one  which  gives  the 
most  adequate  explanation  of  the  known  facts  is  known 

1  For  a  description  of  the  mechanical  testing  of  alloys  and  of  the 
effects  of  work  see  ROSENHAIN,  "Physical  Metallurgy,"  Chapter  XI. 


THE  NON-FERRO US  ALLO  YS  OF  TECHNICAL  IMPORTANCE  73 

as  the  amorphous  cement  theory  which  was  proposed  by 
Beilby  and  has  been  carefully  studied  by  Rosenhain 


FIG.  38A.  —  Moderately   worked    Muntz   metal    (Cu 
(O'Daly.) 


-Zn  40%).     75  X. 


1 

m 


FIG.  38B. — Spe 


;r  hard  drawing. 


and  his  associates.  This  theory,  now  generally  accepted, 
assumes  that  when  a  metal  is  subjected  to  such  an 
amount  of  strain,  either  tensile  or  compressive,  that  its 


74 


PRINCIPLES  OF  METALLOGRAPHY 


elastic  limit  is  exceeded,  the  crystals  become  elongated 
not  by  a  simple  stretching  of  the  material  but  by  a  slip- 
ping taking  place  along  certain  of  the  crystal  planes. 


FIG.  40. 


That  the  elongation  of  the  crystals  is  very  marked  under 
certain  conditions  is  shown  in  the  photomicrographs, 
Figs.  38,  A  and  B.  The  changes  taking  place  within  the 


7  HE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  75 

crystal  are  shown  in  Figs.  39  and  40  sketched  from 
Rosenhain's  photographs  which  represent  a  piece  of  soft 
iron  before  and  after  straining.  The  dark  lines  crossing 
the  crystals  in  Fig.  40  are  due  to  the  fact  that  the  surface 
is  no  longer  plane  but  is  covered  with  a  large  number  of 


8    S    £„»  *    S     , 


D 

A   Before  straining  B  After  straining 

FIG.  41. — Sketch    showing  the  way  in  which  slip   bands  are  produced  in 
strained    metal.     (Rosenhain.) 

microscopic  ridges  formed  by  the  displacement  of  many 
crystal  layers.  These  dark  lines,  or  bands,  are  called 
slip  bands  and  are  characteristic  of  overstrained  metal  (see 
also  Fig.  48,  p.  87).  The  sketches,  Figs.  41  and  42,  illus- 


FIG.  42. — Sketch  showing  the  optical  reason  for  the  appearance  of  slip  bands. 
(After  Rosenhain.) 

trate  the  probable  nature  of  slipping  and  the  optical  reason 
for  the  dark  slip  bands.  Rays  "A"  (Fig.  42)  striking  the 
horizontal  surfaces  are  reflected  back  into  the  eyepiece 
and  produce  light  bands.  Rays  "B"  strike  the  oblique 
surfaces,  are  reflected  out  of  the  field  and  cause  the  black 


76  PRINCIPLES  OF  METALLOGRAPHY 

lines  or  bands.  Granting  that  slipping  does  occur  along 
crystal  planes,  it  is  easy  to  believe  that  the  rubbing  of 
the  surfaces  wholly  destroys  the  crystalline  character 
of  an  extremely  thin  layer  of  the  metal,  producing  what 
has  been  called  an  amorphous  cement  or  amorphous 
binding  material  between  the  displaced  layers.  Since 
it  may  be  assumed  that  the  slipping  would  first  occur 
along  those  planes  where  the  crystalline  cohesion  was 
least,  it  would  naturally  follow  that  a  greater  tension 
would  need  to  be  applied  to  cause  additional  elongation. 
This  fact,  together  with  the  belief  that  the  amorphous 
material  is  both  harder  and  stronger  than  the  crystalline 
form,  would  account  for  the  increased  hardness  and  ten- 
sile strength  of  material,  which  has  been  subjected  to 
mechanical  work.  As  the  amorphous  material  has  no 
planes  along  which  slipping  can  take  place  to  relieve  an 
imposed  strain,  it  follows  that  a  sudden  shock  is  apt  to 
cause  it  to  fracture.  The  same  fact  limits  the  amount 
of  mechanical  work  which  can  be  done  on  a  metal  before 
it  becomes  necessary  to  cause  a  partial  recrystallization 
of  the  amorphous  material  by  annealing.  The  annealing 
of  metals  having  an  excessive  amount  of  amorphous 
material  has  the  function  of  decreasing  the  brittleness 
and  reducing  the  strain  hardness. 

Etching  of  Copper  Alloys. — The  most  effective  reagent 
for  alloys  rich  in  copper  is  ammoniacal  hydrogen  peroxide. 
For  ordinary  brasses  containing  70  per  cent,  copper  and 
30  per  cent,  zinc  the  proportions  should  be  approximately 
one  part  of  3  per  cent,  hydrogen  peroxide  to  five  parts  of 
strong  ammonia  (sp.  gr.  0.90).  The  amount  of  hydrogen 
peroxide  must  be  increased  with  higher  percentages  of  cop- 
per and  decreased  as  the  copper  decreases.  The  ratio  of 
peroxide  to  ammonia  should  be  about  1  to  10  for  use 
with  alloys  of  the  Muntz  metal  type  (Cu  60  per  cent.,  Zn 
40  per  cent.).  The  reagent  does  not  keep  and  it  is 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  77 

essential  for  success  that  it  should  be  prepared  immediately 
before  use.  The  mixture  is  applied  to  the  polished  sur- 
face by  swabbing  with  cotton  soaked  in  the  liquid,  after 
which  the  surface  is  washed  in  running  water  and  again 
treated  for  a  few  seconds  with  the  etching  reagent. 
Alternate  treatments  with  alkaline  peroxide  and  water 
will  soon  develop  the  crystal  structure  in  such  a  way 
that  the  finest  details  become  visible.  This  method  of 
etching  requires  [a  little  experience  but  the  results  will 
be  found  to  repay  any  time  spent  in  practice  with  it. 
Dark,  overetched  surfaces  are  commonly  due  to  excess 
of  ammonium  hydroxide,  while  surfaces  lacking  in  detail 
are  usually  caused  by  too  much  peroxide  although  this 
lack  of  contrast  may  be  due  to  severe  overstrain, 
a-brass  (p.  82)  is  colored  buff  or  brown  by  the  mixture 
while  0-brass  (p.  82)  is  generally  yellow. 

Ammonium  persulphate,  (NH^SaOg,  in  strong  am- 
monia (1  gr.  per  20  c.c.  )  is  often  used  to  identify  /3-brass 
which  it  attacks  more  readily  than  it  does  a-brass. 

Nitric  acid  (sp.  gr.  1.20)  is  used  for  rapid  development 
of  the  crystal  structure.  The  resulting  etched  surface 
shows  strong  contrast  but  the  details  are  not  so  sharply 
denned  as  with  the  peroxide  and  the  alloy  is  not  so  satisfac- 
tory to  photograph. 

Ferric  chloride  solution  (p.  32)  is  effective  for  use  with 
arsenical  brass. 

Bronze.  Copper-tin. — The  equilibrium  diagram  of  the 
copper-tin  alloys  is  very  complex  as  Fig.  43  shows,  con- 
sisting of  one  definitely  established  compound  Cu3Sn 
and  five  solid  solutions  in  which  it  is  probable  that  the 
compounds  CuSn,  Cu5Sn2,  Cu4Sn  and  Cu5Sn  exist,  al- 
though this  fact  has  not  been  fully  established. 

The  important  tin  bronzes  are  practically  all  included 
in  that  section  of  the  diagram  in  which  the  percentage  of 
tin  is  less  than  30  per  cent,  or,  in  other  words,  the  desirable 


78 


PRINCIPLES  OF  METALLOGRAPHY 


properties  are  associated  with  the   a  and   /3  crystals. 
Four  classes  of  bronzes  are  of  importance. 

1.  Coinage  bronze  containing  96  to  92  per  cent,  copper 
is  used  largely  in  the  production  of  "copper"  coins  and 
medals,  the  small  amount  of  tin  present  increasing  the 
hardness  and  wearing  qualities  of  the  copper. 


100%  Sn 


Composition 
FIG.  43. — Copper-tin  diagram. 


2.  Gun  metal  and  gear  bronze  vary  in  composition 
from  92  to  88  per  cent,  copper.  Gun  metal  is  no  longer 
used  in  the  manufacture  of  ordnance  but  is  often  used 
where  strong,  heavy  castings  are  to  be  made.  In  order 
to  increase  the  fluidity  of  the  metal  and  make  the  casting 
operation  simpler,  a  small  amount  of  zinc  is  frequently 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  79 

added.  A  standard  alloy  of  this  class  has  the  composi- 
tion 88  per  cent,  copper,  10  per  cent,  tin  and  2  per  cent, 
zinc.  This  is  usually  known  as  Government  Bronze, "  G  " 
Metal  or  simply  "88,  10  and  2."  It  has  a  fairly  high 
tensile  strength,  32—38,000  pounds  per  square  inch,  and  is 
often  used  where  pressures,  steam  or  hydraulic,  are  to  be 
met,  or  for  bearings  subject  to  heavy  loads. 

Another  bronze  of  the  same  class  consists  of  89  per 
cent.  Cu  and  11  per  cent.  Sn  and  is  very  generally  used, 
under  the  name  of  English  gear  bronze  in  the  manu- 
facture of  heavy  gears.  A  brilliant,  mirror  surface  is 
developed  at  the  contact  between  the  teeth  of  the  gear 
and  the  driving  mechanism  and  an  excellent  bearing  and 
wearing  surface  results. 

3.  Bearing  Bronzes. — These  alloys  vary  from  87  to 
81  per  cent,  copper  and  usually  contain  one  or  more 
elements  in  addition  to  the  tin.  The  best  example  of 
this  class  is  the  bearing  bronze  to  which  phosphorus 
(in  the  form  of  phosphor  copper)  and  lead  have  been 
added.  These  alloys,  called  phosphor  bronzes,  are  of 
two  classes,  those  to  which  phosphorus  is  added  only  as  a 
deoxidizer  and  those  in  which  an  excess  is  present  to  act 
as  a  hardener.  In  the  first  class  the  function  of  the  phos- 
phorus is  simply  to  increase  the  strength  and  ductility  of 
the  alloy  by  removing  the  Cu20  eutectic  and  other  oxides. 
In  many  cases,  in  spite  of  the  marked  improvement  in 
physical  properties,  the  actual  amount  of  phosphorus  is 
negligible.  In  the  second  class,  the  phosphorus,  even 
though  present  in  small  quantities,  usually  less  than  1 
per  cent.,  forms  extremely  hard  particles  of  Cu3P,  too 
brittle  in  themselves  to  be  used  in  a  bearing  but  forming 
and  excellent  non-abrasive  skeleton  in  the  strong,  tough 
bronze  matrix.  Lead  is  frequently  added  to  bronze  in 
small  amounts  and,  as  it  is  an  insoluble  constituent,  is 
found  fairly  uniformly  distributed  throughout  the  metal 


80 


PRINCIPLES  OF  METALLOGRAPHY 


in  the  form  of  drops,  Fig.  44,  A.    Lead  gives  to  the  metal 
two  valuable  characteristics.     It  makes  it  more  easily 


« 


A.  Unetched  bronze  showing  lead  drops. 


B.  Etched  to  show  dendritic  structure. 
FIG.  44. — Phosphor  bronze  with  4  per  cent,  of  lead.     75  X. 

machined   and,    to    a   certain   extent,    self   lubricating 
because  of  the  soft,  greasy  character  of  the  suspended 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  81 

lead.  The  lead  drops  are  sources  of  weakness  in  an 
otherwise  strong  metal  so  that  the  amount  x>f  lead  must 
be  carefully  adjusted  to  fit  the  conditions  under  which  the 
bearing  is  to  be  used.  For  most  purposes  the  lead  con- 
tent is  less  than  2  per  cent.,  although  in  the  plastic 
bronze,  previously  mentioned  (p.  9),  it  may  reach  50 
per  cent. 

4.  Bell  Metals. — These  alloys  contain  from  80  to  75  per 
cent,  copper  and  are  of  especial  interest  because  they  must 
be  worked  either  at  a  temperature  above  dull  redness  or  the 
hot  metal  must  be  suddenly  chilled  (quenched)  and  then 
worked  cold.  Reference  to  the  diagram  (Fig.  43)  will 
show  that  work  is  done  in  both  cases  on  the  jS-solid 
solution.  In  the  first  case,  the  work  is  done  while  the 
alloy  is  in  the  /3-temperature  range  and,  in  the  second  case, 
the  sudden  chill  retains  the  bronze  in  the  condition  in 
which  it  existed  at  the  higher  temperature  (seep.  57). 
With  the  increase  in  tin  to  more  than  25  per  cent.,  the 
brittleness  becomes  so  great  that  the  alloys  are  handled 
only  with  difficulty  and  are  used  exclusively  for  decorative 
purposes  where  the  material  is  not  subjected  to  strain  or 
shock. 

Brass. — The  copper-zinc  alloys  are  the  most  important 
of  the  copper  alloys  because  they  are  relatively  inexpen- 
sive as  compared  to  tin  bronze.  The  diagram  for  the 
brasses  is,  like  that  of  the  bronzes,  very  complex,  consisting 
of  six  series  of  solid  solutions  which  probably  contain,  as 
in  the  other  case,  definite  compounds  (Fig.  45). 

The  7-solid  solution,  which  begins  to  'be  formed  when 
the  percentage  of  zinc  is  increased  above  50  per  cent., 
probably  contains  the  compound  Cu2Zn3  and  is  so  brittle 
that  alloys  in  which  it  occurs  are  practically  valueless 
except  for  decorative  purposes  where  strength  and  duc- 
tility are  not  required.  This  limits  the  technically  im- 
portant brasses  to  three  classes:  a-brass,  from  0  to  36 


82 


PRINCIPLES  OF  METALLOGRAPHY 


per  cent,  zinc;  a  +  |8-brass,  from  36  to  about  42  per  cent, 
zinc ;  and  /3-brass,  from  42  to  about  50  per  cent,  zinc,  at 
which  point  the  tensile  strength  and  ductility  both  drop 
to  nearly  zero.  The  following  curves  show  the  relation- 
ships between  tensile  strength  properties  and  metallo- 
graphic  constitution  of  cast  copper-zinc  alloys  (Fig.  46). 

It  will  be  seen  that  the  strength  of  cast  brass  increases 
from  about  28,000  pounds  per  square  inch  with  pure 
copper  to  more  than  60,000  pounds  per  square  inch  with 


Composition 
FIG.  45. — Copper-zinc  diagram.     (Shepard.) 

45  per  cent,  zinc  (pure  0),  so  that  by  choosing  the  com- 
position, any  desired  strength  within  these  limits  may 
be  obtained.  It  must  be  clearly  understood,  at  this 
point,  that  the  figures  just  given  are  not  absolute,  but 
relative,  as  the  physical  properties  vary  within  fairly  wide 
limits,  even  with  cast  material,  depending  on  various 
factors  such  as  the  shape  of  the  cast  piece,  the  material 
of  which  the  mould  is  made  and  the  rate  at  which  the  metal 
cools. 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  83 


The  actual  number  of  commercial  brasses  is  very  great 
but,  for  convenience,  they  may  be  grouped  in  a  few  classes. 
The  cost  decreases  with  the  increase  in  the  per- 
centage of  zinc  so  that  very  high  copper  alloys  are  not 
much  used. 

1.  Gilding  Metal  or  Jewelry  Brass. — This  contains 
from  1  to  20  per  cent,  zinc  and  is  used  under  various 
trade  names  in  the  manufacture  of  cheap  jewelry.  The 
color  of  some  of  the  alloys  in  the  group  is  not  unlike  that 
of  standard  gold. 


100  j{  Cu  95 


86 


55 


50         45 


75          70         65 
Composition 

FIG.  46. — Relation  between  chemical  composition  of  copper-zinc  alloys  and 
tensile  strength.     (After  Johnson— J.  Inst.  Metals,  xx,  233.) 

2.  Dutch  Metal. — These  alloys  contain  between  20 
and  25  per  cent,  zinc,  are  very  malleable  and  are  used 
largely  in  the  hammered    form  as  substitutes  for  gold 
leaf. 

3.  Brass  for  Cold  Working.— The  group  in  which  the 
zinc  varies  from  27  to  35  per  cent,  includes  by  far  the 
larger   number   of   the   technically   important   brasses. 
This  range  of  compositions  lies  at  the  zinc  rich  end  of  the 
a-brass  field  and,  therefore,  includes  the  alloys  of  high 
tensile  strength  coupled  with  maximum  ductility.     In 


84  PRINCIPLES  OF  METALLOGRAPHY 

this  class  are  found  the  alloys  used  for  sheet  metal, 
tubes,  wire,  cartridge  cases  and  other  articles  which  are 
to  be  subjected  to  severe  mechanical  work. 

4.  Muntz  metal  and  similar  alloys  contain  from  37 
to  45  per  cent,  zinc  and  include  the  a  and  /3-  and  the  pure 
^-brasses.  Pure  /3-brass  does  not  exist  in  copper-zinc 
alloys  which  have  been  cooled  slowly  from  a  high  tem- 
perature but  may  be  obtained  as  a  perfectly  homogene- 
ous solid  solution  by  quenching  an  alloy  with  60  per 
cent,  copper  from  a  temperature  of  800°C.  Owing  to 
the  fact  that  normally  cooled  alloys  of  the  Muntz  metal 
type  are  composed  of  two  components,  a  and  ft,  they  are 
usually  worked  hot  so  that  the  metal  may  be  in  its  homo- 
geneous condition.  Pure  /3-brass  shows  little  or  no 
tendency  to  twin  even  after  it  has  been  worked  and 
annealed.  It  is  colored  yellow  by  NH4OH  and  H2O2 
and  may  be  distinguished  in  this  way  from  a-brass  which 
under  similar  treatment  becomes  distinctly  brown.  A 
mixture  of  NH4OH  and  ammonium  persulphate  is 
often  successfully  used  in  the  examination  of  Muntz 
metal  as  the  /3-brass  is  colored  yellow  while  the  a-brass 
is  practically  unaffected  by  a  short  treatment  with  this 
reagent.  The  characteristic  appearance  of  a-brass  in  a 
ground-mass  of  untwinned  /3-brass  is  shown  in  the  follow- 
ing photographs,  Fig.  47,  A,  B,  C,  and  D,  the  differ- 
ences in  the  size  of  the  a-masses  being  due  to  differences 
in  heat  treatment  and  chemical  composition. 

Alloys  of  the  Muntz  metal  type  are  fairly  resistant 
to  corrosion  by  salt  water  if  the  a  and  ft  crystals  are 
small  and  intimately  mixed.  They  are  used  to  a  con- 
siderable extent  in  the  sheathing  of  wooden  ships,  for 
condenser  tubes  and  for  other  purposes  where  the  lesser 
ductility  is  not  a  serious  objection.  Because  of  the 
cheapness  of  zinc  the  Muntz  metal  alloys  are  sometimes 
substituted  for  the  more  expensive  a-brass.  These 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  85 


A.  Cast  Muntz  metal  (150  X). 


B.  Muntz  metal  annealed  at  750°  and 
quenched.  Chiefly  /3-brass.  75  X.  (Johnson 
and  Jermain.) 


C.    Muntz    metal    annealed    at  750°   and  D.  Muntz  metal  annealed  at  750°  and  cooled 

ooled  in  air.     75  X .     (Johnson  and  Jermain.)       in  furnace.     Shows  brown  islands  of  a-brass  in 

a    matrix    of   /3-brass.      75 X-      (Johnson   and 
Jermain.) 
FIG.  47. — Muntz  metal. 


86  PRINCIPLES  OF  METALLOGRAPHY 

alloys  may  be  made  much  less  readily  corroded  by  the 
addition  of  about  1  per  cent,  tin  as  in  Naval  Brass. 

5.  Brass   Solder. — For  brazing  iron,  the  solder  con- 
tains 35  per  cent,  of  zinc  while,  for  soldering  brass,  the 
alloy  of  50  per  cent,  is  most  commonly  used. 

6.  White   Brass. — When   the   percentage   of   zinc   is 
more  than  50,  the  resulting  alloys  become  increasingly 
light  in  color  and  are  very  brittle.     These  alloys  are 
known  as  the  white  brasses  and  are  used  only  for  orna- 
mental castings. 

a-Brass. — The  most  important  single  alloy  is  that  whose 
composition  is  very  near  70  per  cent,  copper  and  30  per 
cent.  zinc.  It  possesses  great  ductility,  about  56  per  cent, 
elongation  and  a  tensile  strength  when  cast  of  30,000 
to  35,000  pounds  per  square  inch,  a  strength  which  is 
greatly  increased  by  mechanical  work. 

One  of  the  most  important  uses  of  this  "  70-30"  brass 
is  in  the  production  of  shell  or  cartridge  cases  which,  in 
the  process  of  manufacture,  are  subjected  to  severe 
mechanical  work.  Practically  all  brass  shell  cases  from 
those  used  for  the  small  revolver  to  the  large  cases  used 
in  naval  guns  are  made  by'a  series  of  punching  and  draw- 
ing operations.  As  was  stated  on  p.  72,  this  working 
produces  an  elongation  of  the  crystal  grains  and  a  marked 
hardening  and  increase  in  brittleness  of  the  metal.  The 
following  photograph,  Fig.  48,  is  of  interest  in  showing 
the  value  of  metallographic  as  well  as  chemical  control  of 
cold-worked  brass.  The  chemical  analysis  is  excellent 
but  the  strain  has  been  enough  to  produce  great  distor- 
tion of  the  crystal  grains  and  the  production  of  numerous 
slip  bands.  This  strained  condition  can  be  wholly 
relieved  by  suitable  annealing.  The  striking  feature  of 
cold-worked  a-brass  which  has  been  annealed  is  the  pro- 
duction of  twin  crystals  characterized  by  alternate  dark 
and  light  bands  (Fig.  50,  C) .  These  twins  may  be  so  small 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  87 

as  to  be  scarcely  visible  at  a  magnification  of  seventy-five 
diameters  or  they  may  be  so  large  that  with  the  same 


FIG.  48. — Badly  strained  a-brass  showing  distorted  crystals  and  many  slip 
bands.     75  X. 


i-< 


i    .  .j   I-?*  ^  •-**,      *-» 

*:'*V: '-; 

FIG.  49. — Cartridge   brass  as  cast.     75  X. 

n  agnification  a  single  crystal  may  cover  the  entire  field 
of  vision  of  the  microscope.     These  differences  in  crystal 


88  PRINCIPLES  OF  METALLOGRAPHY 

size  are  shown  in  Figs.  49  and  50 A,  B,  C,  D,  which  are 
all  a-brasses  of  the  same  chemical  composition  (70  per 
cent,  copper  and  30  per  cent,  zinc),  but  differ  in  the 
amount  of  mechanical  and  heat  treatment.  Although 
it  may  be  said  in  a  general  way  that  the  higher  the  anneal- 
ing temperature  the  larger  the  crystal  grains,  other  con- 
ditions being  equal,  it  is  also  true  that  the  crystal  size 
depends  on  the  amount  of  cold  work  to  which  the  brass 
has  been  subjected.1  This  means  that  for  each  sample 
of  cold- worked  brass  there  is  an  annealing  temperature 
which  will  produce  crystals  of  the  desired  size,  this  tem- 
berature  depending  on  the  extent  to  which  the  brass  has 
been  deformed  by  mechanical  work.  Fine  crystal  grains 
indicate  increased  tensile  strength  and  hardness  with  de- 
creased ductility  while  large  crystals  are  always  accom- 
panied by  softness,  lower  tensile  strength  and  greater 
ductility. 

This  connection  between  crystal  size  and  physical 
properties  has  led  to  the  introduction  of  definite  grain 
size  requirements  in  many  specifications,  not  only  for 
brass  and  bronze  but  also  for  steel.  A  convenient  means 
of  studying  grain  size  has  been  proposed  by  Jeffries2  and 
is  recommended  by  the  American  Association  for  Testing 
Materials.  The  method  consists  in  projecting  the  mag- 
nified image  of  the  specimen  onto  a  ground  glass  plate 
on  which  has  been  inscribed  a  circle  79.8  millimeters 
in  diameter  (area  =  5000  sq.  mm.).  The  ground  glass 
is  placed  with  its  ground  surface  toward  the  specimen 
and  on  the  outer,  smooth  surf  ace  the  number  of  whote  crys- 
tals included  in  the  circle  is  counted.  This  may  be  done 
conveniently  by  checking  each  crystal  with  a  soft  (glass) 
pencil.  The  number  of  grains  intersecting  the  circum- 

1  MATHEWSON  and  PHILLIPS,  Am.  Inst.  Min.  Eng.,  Feb.,  1916. 

2  ZAY  JEFFRIES,  "Grain  Size  Measurements,"  Met.  and.  Chem.  Engr., 
vol.  xviii,  p.  185. 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  89 


A.    Severely  cold  worked  cartridge  metal  be- 
ginning  to   recrystallize.     75  X. 


B.    Cold   worked  and   annealed.     Small   twin 
crystals  are  visible.     75  X. 


C.    Moderately  cold  worked  cartridge  metal  D.    Moderately  cold  worked  cartridge  brass 

fter    annealing.     75  X-     (The     characteristic       annealed   at  700°  C.     75  X.   (Large  twin  cry s- 
winned  structure  is  very  marked.)  tals  due  to  overheating.) 

FIG.  50. — Brass — 70  %  copper,  30  %  zinc. 


90 


PRINCIPLES  OF  METALLOGRAPHY 


ference  of  the  circle  is  then  counted  and  0.5  of  this 
number,  added  to  the  number  completely  included  in 
the  circle,  gives  a  close  approximation  to  the  total 
number  of  crystals  present.  To  obtain  the  number  of 
grains  per  square  millimeter,  the  crystal  count  is  multi- 
plied by  a  factor  which  depends  on  the  magnification 
used.  The  standard  magnifications,  as  recommended 
by  the  American  Society  for  Testing  Materials,  are,  for 
steels,  50-100-250  and  500  diameters  and  for  non-ferrous 
alloys,  25-75-150  and  250  diameters.  The  multiplying 
factors  are  given  in  the  following  table. 


Diameter  of 
circle  in  milli- 
meters 

Magnification 
used 

Multiplying  factor 
to  obtain  grains  per 
square  millimeter 

79.8 

10 

0.020 

79.8 

25 

0.125 

79.8 

50 

0.500 

79.8 

75 

1.125 

79.8 

100 

2.000 

79.8 

150 

4.500 

79.8 

250 

12.500 

79.8 

500 

50.000 

If  the  grain  size  is  to  be  expressed  as  the  average 
diameter  of  the  crystal  in  millimeters,  or  its  area  in  ju2, 
the  following  formulas  from  Jeffries  paper  will  be 
found  useful : 

z  =  completely  included  grains; 
w  =  boundary  grains; 

x  =  equivalent  number  of  whole  grains  in  5000  sq. 
mm.   (circle  79.8  mm.  in  diameter  or  rectangle 

with  area  of  5000  sq.  mm.) ;  x  =  ~  w  +  z. 

m  =  magnification; 
/  =  multiplying   factor   used   to   obtain   grains   per 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  91 


square  millimeter  (see  table)  ;  /  = 


n  =  number  of  grains  per  sq.  mm.;  n  =  fx 

:  d  = 
1,000,000 


d  =  diameter  of  average  grain  in  mm.  :  d  =  ~F 

Vn 


a  =  area  of  average  gram  in  M2-     a 

One  of  the  most  serious  defects  in  worked  brasses  and 
bronzes  is  the  strained  condition  leading  to  the  formation 
of  what  are  known  as  season  cracks.  Various  articles  of 
cold  worked  brass  may  be  so  badly  strained  and  so 
imperfectly  annealed,  that  storage  for  a  period  of  from 
several  weeks  to  a  number  of  months,  particularly  in  a 
moist  climate,  leads  to  a  spontaneous  break  down  of  the 
strained  metal  and  the  production  of  large  or  small 
season  cracks.  It  happens,  frequently,  that  this  dan- 
gerous condition  is  not  at  all  apparent  even  on  careful 
inspection.  Because  of  its  comparative  frequency,  it 
has  become  the  custom  in  many  instances  to  insist  on  a 
test  for  season  cracking  with  a  specified  number  of 
samples  from  an  entire  lot.  This  can  be  done  effectively 
by  immersion  of  the  suspected  sample  for  4  hours,  in  a 
\Y^  per  cent,  solution  of  mercuric  chloride  or  mercuric 
nitrate.  The  season  cracking  phenomenon  is  greatly 
accelerated  by  this  treatment  and  the  tendency  to  crack 
at  once  disclosed. 

It  is  essential  that  a-brass  which  is  to  be  exposed  to 
severe  mechanical  treatment  should  be  free  from  bis- 
muth, antimony,  iron  and  lead  as  all  are  sources  of 
weakness.  For  brass  which  receives  only  a  moderate 
treatment,  small  percentages  of  iron  or  lead  will  not  be 
dangerous.  Iron  gives  to  the  alloy  increased  strength 
coupled  with  increased  hardness  and  decreased  ductility, 
while  lead  acts,  as  it  does  with  bronze,  to  reduce  the 


92  PRINCIPLES  OF  METALLOGRAPHY 

tensile  strength  but  to  make  the  brass  far  more  readily 
machined.  Bismuth  and  antimony  tend  to  form  brittle 
envelopes  around  the  a-crystals  and  to  destroy  the  duc- 
tility that  makes  cold  work  possible. 

For  special  purposes,  small  percentages  of  other  ele- 
ments are  added  to  the  brass  alloys.  The  use  of  lead 
has  been  mentioned  as  improving  the  machining  quali- 
ties though  reducing  the  tensile  strength,  but  only  when 
the  percentage  of  lead  is  very  low  (less  than  0.5  per  cent.) 
can  the  brasses  be  worked  hot.  Tin,  when  added 
in  small  quantities  increases  the  hardness  of  brass 
but  causes  a  marked  decrease  in  ductility.  When  added 
to  70-30  brass  in  amounts  from  1  to  1.5  per  cent.,  an 
alloy  which  is  very  resistant  to  sea  water,  Admiralty 
metal,  is  formed.  The  addition  of  manganese  in  amounts 
less  than  4  per  cent.,  gives  to  the  brasses  very  desirable 
properties.  It  is  usually  added  to  brasses  of  the  Muntz 
metal  type,  together  with  small  amounts  of  tin,  iron  and 
aluminum.  An  alloy  of  this  general  type,  called,  unfor- 
tunately, manganese  bronze  when  it  should  be  man- 
ganese brass,  is  much  used  in  making  propeller  blades, 
rudders,  ship  fittings  exposed  to  sea  water,  and  for  other 
engineering  purposes  requiring  a  strong,  non-corrodible 
alloy.  Aluminum,  when  added  in  very  small  amounts, 
increases  the  fluidity  of  molten  brass  to  a  marked  degree, 
rendering  the  casting  operation  easier  and  producing 
cleaner  castings.  It  materially  increases  the  strength 
of  the  brass  but  rapidly  reduces  its  ductility  so  that 
the  amount  added  should  never  exceed  3  per  cent. 

Aluminum  Bronze. — Another  technical  copper  alloy 
of  great  importance  is  the  alloy  with  aluminum  known 
as  aluminum  bronze.  The  diagram,  Fig.  51,  is  not 
unlike  the  tin-bronze  and  brass  diagrams  in  its  general 
character  and  complexity.  The  only  alloys  of  technical 
importance,  however,  lie  in  the  a-field  (copper  solid 


THE  NON-FERROUS  ALLOYS  OF  TECHNICAL  IMPORTANCE  93 


solution  varying  from  0  to  about  11  percent,  aluminum) 
and,  at  the  opposite  side,  in  the  T?  field,  which  is  a  series 
of  solid  solutions  of  copper  in  aluminum,  saturated  at 
about  10  per  cent,  copper.  The  light  alloys  of  aluminum 
with  copper  have  already  been  mentioned,  p.  69.  The 
alloys  in  the  a-field  have  remarkable  physical  properties, 
the  addition  of  aluminum  causing  a  striking  increase  in 
tensile  strength.  In  castings,  for  example,  while  30  per 


s,w  SspQZ 
L2J3B32 


FIG.  51. — Aluminum    bronze    diagram.     (Carpenter    and    Edwards,    Gwyer, 
Curry.) 

cent,  'zinc  gives  a  brass  with  a  tensile  strength  of  about 
30,000  pounds  per  square  inch  and  10  per  cent,  tin  will 
give  a  bronze  with  about  40, 000  pounds  tensile  strength, 
the  addition  of  10  per  cent,  aluminum  to  copper  gives  an 
aluminum  bronze  with  a  strength  of  about  70, 000  pounds 
per  square  inch.  Its  elongation  is  about  20  per  cent. ,  nearly 
as  great  as  that  of  brass  and  twice  as  much  as  that  of 
tin-bronze  and  the  alloy  is  considerably  harder  than 
either.  Aluminum  bronze  is  used  in  the  manufacture  of 
castings  requiring  strength  and  toughness,  and  is  espe- 


94  PRINCIPLES  OF  METALLOGRAPHY 

cially  resistant  to  shock  or  to  alternating  stresses. 
It  has  the  added  advantage  that  it  is  from  10  to  15  per 
cent,  lighter  than  the  corresponding  brasses  and  tin 
bronzes.  This  alloy  would  be  more  extensively  used 
if  it  were  not  for  certain  difficulties  in  its  manufacture 
which  are  apt  to  cause  lack  of  uniformity  in  the  finished 
product.  Properly  made  aluminum  bronze  is  a  very 
valuable  alloy. l 

1  Extended  discussion  of  the  technical  non-ferrous  alloys  will  be  found 
in  LAW,  "Alloys  and  Their  Industrial  Applications;"  GULLIVER,  "Metal- 
lic Alloys;"  DESCH,  "Metallography."  References  to  current  practice 
will  be  found  in  the  "Institute  of  Metals"  (British)  and  in  the  "American 
Institute  of  Metals"  which  has  recently  become  affiliated  with  the 
"Institute  of  Mining  Engineers." 


CHAPTER  V 
IRON  AND  STEEL 

The  most  important  applications  of  metallography, 
as  well  as  the  most  difficult,  lie  in  the  uses,  defects  and 
methods  of  heat  treatment  of  iron  and  steel.  The  diffi- 
culties of  the  study  of  this  series  of  iron-carbon  alloys 
may  be  traced  to  various  causes,  among  them  the  fact 
that  in  the  technical  study  we  are  dealing,  even  in 
what  are  known  as  the  "  plain  carbon-"  steels,  not  with 
a  simple  alloy  of  iron  and  carbon  but  with  an  exceedingly 
complex  mixture  of  iron,  carbon,  phosphorus,  manganese, 
sulphur  and  silicon.  While  the  effects  of  the  last  four 
elements,  when  they  are  present  in  small  quantities,  as  is 
usually  the  case,  are  not  comparable  with  the  effects 
produced  by  comparatively  slight  changes  in  carbon 
content,  no  one  of  the  constituents  can  be  wholly  neg- 
lected. Increase  in  any  one  of  them,  above  a  certain  well- 
recognized  maximum,  causes  far-reaching  changes  in  the 
physical  and  metallographic  properties  of  the  metal. 
When  elements  like  chromium,  nickel,  vanadium  or  tungs- 
ten are  added  in  making  the  alloy  steels  (self  hardening,  high 
speed  tool  steel,  etc.),  the  situation  becomes  so  complex 
that  little  is  known  from  an  equilibrium  standpoint. 
An  enormous  amount  of  work  is  yet  to  be  done  in  sys- 
tematizing the  present  knowledge  of  the  properties  of 
the  alloy  steels. 

A  second  factor  which  complicates  the  exact  study  of 
the  iron-carbon  diagram  is  that  the  iron  exists  in  various 
allotropic  forms,  each  one  of  which  has  different  physical 
properties,  notably  magnetic  properties  and  each  one  of 

95 


96  PRINCIPLES  OF  METALLOGRAPHY 

which  varies  in  its  ability  to  dissolve  carbon.  During 
the  cooling  of  chemically  pure  iron  (electro-deposited) 
five  or  six  holds  in  the  curve  have  been  noted  by  various 
investigators  but,  because  of  uncertainties  as  to  the 
possible  effects  of  dissolved  gases,  it  is  generally  assumed 
that  iron  exists  in  three  allotropic  forms;  (1)  7-iron,  stable 
above  900°  and  practically  nonmagnetic;  (2)  /3-iron, 
existing  between  780°  and  900°;  and  (3)  «-iron,  stable 
below  780°  and  strongly  magnetic.1  The  changes  just 
indicated  occur  on  cooling  the  iron  and,  from  the  initial 
of  the  French  word,  "refroidessement,"  are  often  referred 
to  as  the  Ar3,  Ar2  and  ATI  points,  respectively.  The 
changes  take  place  at  slightly  higher  temperatures  on 
heating  and,  in  abbreviation  of  the  word,  "chauffage, " 
are  often  called  the  Ac3,  Ac2  and  Aci  points.  They  are 
also  called  critical  points  or  transformation  points.  As 
the  percentage  of  carbon  is  increased  the  Ar3,  Ac3,  Ar2 
and  Ac2  are  progressively  lowered  until  at  the  tempera- 
ture of  780°  to  790°C.  and  with  the  carbon  content  in- 
creased to  0.85  per  cent,  the  three  critical  points  coincide. 
The  temperature  range  which  includes  all  the  critical 
points  is  called  the  critical  range  and  is  of  great  impor- 
tance in  the  annealing  of  steel.  This  temperature  range 
extends  from  about  800°  to  900°  with  low  carbon  steels, 
decreases  to  an  interval  of  10°  (from  780°-790°C.)  at 
the  composition  0.85  per  cent,  carbon  and  then  increases 
from  this  point  as  the  carbon  content  increases. 

A  third  complication  in  the  iron-carbon  system  is  due 
to  the  fact  that  many  of  the  reactions  involved  occur 
in  the  solid  state  and  therefore,  unless  the  alloys  cool 
with  extreme  slowness,  the  changes  take  place 

XA  group  of  English  metallographists  question  the  existence  of  /8-iron  and 
consider  only  the  a  and  y  forms.  The  great  majority  of  metallog- 
raphists, however,  still  agree  that  thermal  evidence  warrants  belief  in 
the  /8-form. 


IRON  AND  STEEL  97 

incompletely  or  not  at  all.  It  is  also  true  that  in  the 
higher  carbon  range,  from  4.3  per  cent.  C  upward,  the 
equilibrium  relationships  have  never  been  satisfactorily 
settled. 

The  enumeration  of  these  various  difficulties  might 
lead  to  the  impression  that  a  study  of  the  metallography 
of  iron  and  steel  is  a  hopeless  task.  This  is  far  from  true 
as  many  of  the  difficulties  have  been  overcome  and 
the  study  has  been  carried  on  along  so  many  lines  that 
information  of  the  utmost  value  to  the  makers  and  users 
of  iron  and  steel  has  been  obtained.  It  is  also  true  that 
so  much  remains  to  be  done  that  there  is  an  unlimited 
field  of  investigation  for  those  who  have  the  opportunity 
and  the  inclination  to  carry  on  the  work. 

It  is  beyond  the  scope  of  this  book,  nor  is  it  its  purpose, 
to  consider  except  in  a  general  way  the  various  branches 
of  the  metallography  of  iron  and  steel.  For  detailed 
information  the  reader  is  referred  to  one  of  the  larger 
books  dealing  solely  with  this  side  of  the  subject.1 

Classification  of  the  Iron  Carbon  Alloys. — Dealing 
first  with  that  group  of  alloys  consisting  chiefly  of  iron 
with  varying  amounts  of  carbon,  the  metals  are  divided 
into  three  groups,  depending  on  the  carbon  content 
and  method  of  manufacture;  (1)  wrought  iron,  (2)  steel 
and  (3)  pig  or  cast  iron. 

Wrought  iron  contains  normally  less  than  0.3  per  cent, 
carbon  and  is  prepared  by  melting  the  crude  pig 
iron,  as  it  conies  from  the  smelting  furnace,  with 
hematite  (iron  oxide).  The  resulting  pasty  mass  is 
first  hammered  and  then  rolled  to  remove  from  it  most 
of  its  impurities.  The  chief  foreign  substance  is  an  iron 
silicate  slag  which  is  never  wholly  removed  by  the 
squeezing  of  the  rolls  but  becomes  elongated,  giving  to 
wrought  iron  its  characteristic  fibrous  structure,  Fig.  52. 

1  SAUVEUR,  "Metallography  and  Heat  Treatment  of  Iron  and  Steel." 

7 


98  PRINCIPLES  OF  METALLOGRAPHY 

This  material  has  been  largely  replaced  by  the  cheaper 
mild  steel  which  has  many  of  the  same  physical  properties. 
Wrought  iron  is  sometimes  specified,  however,  because 
of  its  easy  welding  and  its  resistance  to  shock. 

Steel  includes  the  alloys  having  less  than  1.7  per  cent, 
carbon  and  the  properties  of  this  group  of  alloys  is  sub- 


FIG.  52. — Wrought   iron. 

ject  to  the  widest  variation,  depending  on  the  method 
of  production,  the  rate  of  cooling,  the  subsequent  heat 
treatment  and  other  factors. 

The  term  iron  includes  the  alloys  from  1.7  per  cent, 
carbon  upward,  usually  not  in  excess  of  4  or  5  per  cent. 

The  Equilibrium  Diagrams. — Many  metallographists 
have  studied  the  alloys  of  iron  and  carbon  in  the  greatest 
detail  and  have  proposed  new  equilibrium  diagrams  or 


IRON  AND  STEEL 


99 


have  suggested  modifications  of  the  first  one.     Though 
differing  somewhat  from  it  in  special  points,  all  of  them, 


with  the  exception  of  the  Upton  diagram,1  resemble  in 
general  outline  the  early  diagram  of  Roberts- Austen.2 

1  UPTON,  /.  Phys.  Chem.,  12  (1908),  506. 

2  ROBERTS- AUSTEN,  Proc.  Inst.  Mech.  Eng.,  1899,   35. 


100  PRINCIPLES  OF  METALLOGRAPHY 

This  is  especially  true  of  the  alloys  in  the  steel  range 
(less  than  1.7  per  cent.  C).  Much  more  uncertainty 
exists  in  the  range  of  the  irons,  largely,  no  doubt,  because 
the  lesser  importance  of  this  group  has  not  warranted 
the  immense  amount  of  study  that  has  been  given  to 
steel. 

A  simplified  and  probably  incomplete  iron-carbon 
diagram  is  given  in  Fig.  53  from  which  most  of  the  im- 
portant general  relationships  can  be  studied.  The 
diagram  is  a  combination  of  eutectic  (p.  11),  solid  solu- 
tion (p.  39),  compound  (p.  49)  and  eutectoid  (p.  57). 
The  valuable  properties  of  steel  depend  to  a  large  ex- 
tent on  the  fact  that,  as  is  always  true  of  reactions  in  the 
solid  state,  the  decomposition  of  the  solid  solution  7  into 
its  components  requires  time  and  can  be  prevented  almost 
wholly  by  a  sufficiently  sudden  cooling.  It  is  evident 
from  the  diagram  that  pure  iron  never  separates  from  a 
liquid  solution  of  carbon  in  iron  but  that  the  solid  which 
first  separates  along  the  line  FeE  is  a  solid  solution  of 
carbon  in  iron  becoming  a  saturated  solution  when  1.7 
per  cent,  carbon  has  been  added.  Referring  to  the 
diagram,  it  will  be  seen  that  all  steels  are  originally 
solid  solutions  of  carbon  in  iron,  the  carbon  content 
varying  from  almost  zero  up  to  the  saturation  point,  1.7 
per  cent.  The  eutectic  E  is  a  mixture  of  the  solid  solu- 
tion B  and  the  solid  which  separates  along  the  line  EC. 
Whether  the  solid  separating  on  this  line  is  actually  the 
definite  compound  (Fe3C)  is  open  to  question  and  will 
be  considered  later. 

Decomposition  of  the  solid  solution  7  takes  place  along 
the  lines  FP  and  BP  which  intersect  at  the  eutectoid 
point  P.  FP  is  a  line  along  which  pure  iron  separates  and 
BP  represents  the  separation  of  the  definite  compound 
Fe3C. 

Consider,  now,  the  changes  which  take  place  when  a, 


IRON:  AN    STSSP;'*  :;  :«  :  :  /.         101 


steel  containing  0.5  per  cent,  carbon  cools  from  the 
molten  state  to  ordinary  temperatures  as  indicated  by 
the  line  xx'  in  Fig.  53.  At  x  the  metal  is  liquid.  When 
the  line  FeE  is  reached  a  solid  solution  begins  to  separate 
and  at  the  temperature  represented  by  the  intersection 
of  xx'wiih  FeB  the  steel  has  wholly  solidified.  Through- 
out the  area  FeFPB,  the  alloy  is  a  solid  solution,  possibly 
of  carbon  in  iron,  but,  more  probably,  of  the  compound 
Fe3C  in  iron.  Along  the  line  FP  a  change  in  the  solid 
state  takes  place  with  a  separation  of  pure  iron  and,  as  a 
result,  a  change  in  the  concentration  of  remaining  solid 
solution  until  it  contains  about  0.85  per  cent,  carbon 
and  has  reached  a  temperature  of  from  680°  to  700°. 
At  this  temperature  and  composition  the  final  change  in 
the  solid  solution  takes  place  with  the  formation  of 
the  eutectoid  P,  an  intimate  mixture  of  the  pure  iron 
and  the  compound  Fe3C.  Steels  containing  less  carbon 
than  that  corresponding  to  the  eutectoid  (0.85  per  cent. 
C)  are  known  as  hypo-eutectoid  steels  while  those  from 
0.85  to  1.7  per  cent.  C  are  the  hyper-eutectoid  steels. 
In  this  latter  range  the  solid  solution  decomposes  along 
the  line  BP  with  the  separation  of  the  compound  Fe3C, 
reducing  the  carbon  content  until  the  eutectoid  point 
P  is  again  reached,  when  the  same  eutectoid  mixture 
as  before  is  formed.  When  complete  equilibrium  has 
been  established,  hypo-eutectoid  steels  consist  of  varying 
amounts  of  pure  iron  imbedded  in  the  eutectoid  P, 
while  the  hyper-eutectoid  steels  are  mixtures  of  Fe3C 
with  the  same  eutectoid.  At  P,  only  the  eutectoid 
will  be  found. 

Incomplete  Transformations.  —  If  the  behavior  of  steel 
on  rapid  cooling  was  as  simple  as  has  just  been  indi- 
cated there  would  be  only  three  classes  of  steels,  (1) 
mixtures  of  iron  and  the  eutectoid,  (2)  the  eutectoid 
itself  and  (3)  mixtures  of  Fe3C  and  the  eutectoid.  Thus 


102         .'. 

the  physical  properties  of  the  steel  could  be  readily 
determined  by  a  knowledge  of  the  physical  properties  of 
the  three  substances  involved.  It  is,  however,  rarely  the 
case  in  practice  that  complete  equilibrium  is  established, 
so  that  by  far  the  larger  number  of  technical  steels  rep- 
resent imperfect  equilibria  due  to  incomplete  trans- 
formations along  the  lines  indicated  in  the  diagram. 
Extremely  rapid  cooling,  such  as  quenching  from  a  high 
temperature  in  liquid  air,  produces  the  unchanged  solid 
solution;  while  either  very  slow  cooling  or  annealing 
for  a  considerable  time  at  a  temperature  just  below  the 
eutectoid  temperature,  will  give  the  eutectoid  mixture. 
Between  these  extremes  are  various  intermediate  transi- 
tion forms  with  varying  physical  properties.  Several 
of  these  intermediate  substances  have  such  characteristic 
microscopic  structure  and  definite  physical  properties 
that  they  have  been  named  as  a  means  of  distinguishing 
them  from  each  other.  These  names  are  not  at  all  de- 
scriptive but  have  a  certain  historical  interest  as  they  are 
based  on  the  names  of  men  who  have  been  leaders  in  the 
development  of  the  science  of  metallography.  The  un- 
decomposed  7-solid  solution  is  called  Austenite,  after 
Roberts- Austen,  one  of  the  pioneers  in  the  metallography 
of  steel.  Following  this  is  the  more  usual  component 
found  in  quenched  steel,  Martensite,  after  the  German 
metallographist  Martens;  next  Troostite,  from  the  French 
chemist  Troost;  Sorbite,  after  Sorby,  and  finally  Pearl - 
ite,  resembling  mother  of  pearl,  the  only  product  of  which 
the  name  is  at  all  descriptive.  The  transition  briefly 
stated  is  from  Austenite  —>  Martensite  —» Troostite  —» Sor- 
bite —>Pearlite.  To  complete  the  naming  of  the  common 
constituents  of  steels,  the  pure  iron  separating  along  the 
line  FP  has  been  called  Ferrite  and  the  compound  Fe3C, 
Cementite,  as  it  is  the  important  constituent  of  those 
steels  which  have  been  hardened  by  the  cementation 


IRON  AND  STEEL  103 

process.  It  is  hardly  necessary  to  say  that  the  changes 
are  not  as  abrupt  as  the  limited  series  of  names  might 
indicate  but  that  there  are  still  other  intermediate  prod- 
ucts to  which  such  combination  names  as  troosto- 
sorbite  and  sorbitic-pearlite  have  been  given.  The  name 
Osmondite,  from  Osmond  the  French  metallographist 
who  was  a  pioneer  in  the  microscopic  study  of  steel, 
is  sometimes  given  to  the  product  at  the  exact  boundary 
between  troostite  and  sorbite  but  is  not  frequently  used 

Etching  of  Steel  and  the  Microscopic  Appearance  of 
Its  Constituents. — Many  etching  reagents  have  been 
suggested,  some  of  them  extremely  complex  mixtures, 
but  for  most  purposes  three  or  four  different  solutions 
will  be  found  sufficient. 

Nitric  Acid  and  Alcohol. — The  most  commonly  used 
reagent  is  a  solution  containing  4  c.c.  of  concentrated 
nitric  acid  (sp.  gr.  1.42)  in  96  centimeters  of  ethyl  alcohol. 
This  is  used  by  immersing  the  specimen  for  about  10 
seconds  and  agitating  the  liquid  constantly  to  prevent 
the  retention  of  gas  bubbles  on  the  polished  surface. 
After  treatment,  the  specimen  is  washed  thoroughly  in 
running  water  and  dried  either  by  patting  gently  with 
soft  linen  or  cotton,  or  by  means  of  a  blast  of  air. 

Picric  Acid. — For  low  carbon  steels  a  solution  of 
5  grams  of  picric  acid  in  95  c.c.  of  alcohol  is  frequently 
used. 

Alcoholic  Hydrochloric  Acid. — Martens  and  Heyn 
recommend  the  use  of  a  solution  containing  1  part  of 
hydrochloric  acid  (sp.  gr.  1.19)  in  each  100  parts  of  alco- 
hol. The  reagent  acts  much  more  slowly  than  alcoholic 
nitric  acid,  requiring  about  one  minute,  but  the  results 
are  excellent. 

Special  Reagents.  Copper  Ammonium  Chloride. — For 
examination  of  the  specimen  without  the  microscope 
(macroscopic  examination),  and,  especially,  for  the  pur- 


104  PRINCIPLES  OF  METALLOGRAPHY 

pose  of  locating  segregated  areas  in  large  specimens,  a 
solution  of  8  parts  of  copper  ammonium  chloride  in  100 
parts  of  water  is  used.  This  reagent  is  suitable  only  for 
plain  carbon  steels  and  must  be  applied  by  immersing 
the  specimen. 

Iodine  Solution. — A  solution  containing  6  parts  of 
iodine  in  100  parts  of  alcohol  is  used  for  macroscopic 
examination  and  is  applied  by  swabbing  the  polished 
surface  with  cotton  soaked  in  the  solution.  The  opera- 
tion is  continued  for  five  minutes,  a  fresh  portion  of 
reagent  being  added  as  soon  as  the  iodine  color  produced 
by  the  preceding  treatment  has  disappeared.  The 
iodine  reagent  must  be  prepared  immediately  before  use. 
It  is  effective  in  showing  excessive  slag,  phosphorus  segre- 
gations and  other  irregularities. 

Kourbatoff's  Reagent  for  Cementite. — Cementite  is  not 
affected  by  the  usual  reagents  but  is  colored  black  or 
brown  by  immersion  from  three  to  five  minutes  in  a  boiling 
solution  of  sodium  pier  ate  in  sodium  hydroxide.  The 
reagent  contains  2  parts  of  sodium  picrate  in  98  parts 
of  a  25  per  cent,  solution  of  sodium  hydroxide.  Ferrite 
is  unaffected  by  this  reagent  so  that  Kourbatoff's 
mixture  furnishes  a  sure  means  of  distinguishing  between 
these  two  components. 

Time  Required  for  Etching. — With  all  these  reagents 
the  time  required  varies  somewhat  with  the  nature  of 
the  steel  under  examination.  If  pearlite  requires  10 
seconds  immersion,  as  it  will  with  alcoholic  nitric  acid, 
sorbite  will  need  7  or  8  seconds,  martensite  about  5 
seconds  and  troostite,  which  is  most  sensitive  to  chemical 
reaction,  will  take  only  2  or  3  seconds  for  the  develop- 
ment of  its  structure. 

Occurrence  and  Physical  Properties  of  the  Constitu- 
ents of  Steel.  General. — In  dealing  with  slowly 
cooled  steels  in  complete  equilibrium  it  is  only  necessary 


IRON  AND  STEEL 


105 


to  consider  the  physical  properties  of  the  three  possible 
constituents,  (1)  ferrite,  (2)  pearlite,  and  (3)  cementite. 
These  are  shown  in  the  following  table  (Sauveur). 


Constituents 

Tensile  strength  in 
>ounds  per  square 
_        incn 

Elongation, 
per  cent,  in 
2  inches 

Hardness 

Ferrite  

About    50  000 

40  +              Soft 

Pearlite  

About  125  000 

10  +             Hard 

Cementite  

5000  (?) 

0           Very  hard 

and  brittle 

180     90 

100     80 

M 
5  WO     70 
1     s 

1    s 

EFFE 

STOFC 
j 

A. 

ARBON  ON  PHYSICAL  PROPER 

nnealed  just  above  -  Ac3 
Tensile  Strength 
Elastic  Limit  Yield  Point 
Elongation 
Contraction  of  Area 

TIES 

B- 
C- 
D- 

D 

\ 

\ 

I    3 
1    1 

I100!50 

°        a 
|  SO-40 

H  60^30 

I    * 

40     20 
20     10 
0 

\ 

£_ 

A 

^^^ 

~y( 

X 

^ 

\ 

/ 

S^fr 

\ 

A 

^ 

>^ 

-\ 

J- 



x^ 

•^   •^ 

\ 

/ 

B 

^^. 

^/ 

x 

S 

1.00         1.20 

FIG.  54. — Diagram  showing  effect  of  carbon.     (Ajter  J.  H.  Nead,  Am.  Ins*. 
Min.  Eng.   (1916),  2341.) 

As  every  annealed  steel  is  a  mixture  of  two  of  these  three 
constituents,  it  is  to  be  expected  that  the  physical  prop- 
erties can  be  fairly  closely  predicted  from  the  chemical 


106  PRINCIPLES  OF  METALLOGRAPHY 

composition.  The  tensile  strength,  for  example,  in- 
creases from  about  50,000  pounds  per  square  inch  with 
pure  iron,  to  125,000  pounds  per  square  inch  with  pear- 
lite  (0.85  per  cent.  C),  and  then  decreases  as  the  amount 
of  cementite  increases.  In  the  same  way  the  ductility, 
as  represented  by  the  percentage  elongation,  decreases 
from  that  of  pure  iron  and  becomes  very  low  indeed 


A  B 

FIG.  55. — Pearlite  and  ferrite. 
A  =  123  X.     B  =  1650  X. 

when  the  carbon  content  is  more  than  1.20  per  cent 
owing  to  the  increase  in  brittle  cementite.  These 
general  relations  of  pearlitic  (fully  annealed)  steels 
are  shown  in  the  sketch,  Fig.  54,  and  the  microscopic 
appearance  is  given  in  Figs.  55  and  56.  The  former 
illustrates  a  hypo-eutectoid  steel,  the  latter  is  typical 
of  the  hyper-eutectoid  class.  The  thumb  print  struc- 
ture in  each  case  is  the  eutectoid  pearlite.  By  heat- 


IRON  AND  STEEL  107 

ing  pearlitic  steels  for  a  number  of  hours  at  600°- 
700°  the  laminated  structure  gradually  disappears,  due 
to  the  coagulation  of  the  layers  of  cementite  in  the 
eutectoid,  with  the  formation  of  spherical  masses. 
This  operation  is  known  as  spheroidizing  and  the  cemen- 
tite as  spheroidized  cementite. 


FIG.  56. — Cementite  and  pearlite  1650  X-     (The  white  mass  is  cementite  and 
the  dark  eutectoid  pearlite.) 

The  slowly  cooled  (pearlitic)  steels  have  many  indus- 
trial uses.  Very  low  carbon  steel,  or  extra  mild  steel, 
containing  less  than  0.1  per  cent.  C  is  used  for  articles 
which  must  be  readily  worked  like  rivets  or  horseshoe 
nails  and  is  also  used  for  material  which  is  to  be 
subjected  to  cementation  (see  p.  117).  Low  carbon,  or 
mild  steel,  with  carbon  up  to  0.25  per  cent,  may  be 


108  PRINCIPLES  OF  METALLOGRAPHY 

used  for  screws,  bolts,  agricultural  implements,  sheets, 
wire  and  structural  steels,  though  much  of  this 
material,  due  to  treatment  in  the  process  of  manu- 
facture, is  not  strictly  pearlitic.  Steels  containing  from 
0.25  per  cent,  to  about  0.60  per  cent,  carbon  form 
the  class  usually  known  as  medium  high  carbon  or 
half  hard  steel.  As  the  carbon  content  increases  the 
use  of  completely  pearlitic  steel  is  decreased.  Wholly 
or  partially  pearlitic  steels  in  this  range  are  used  for 
castings,  shafting,  piston  rods  and  cylinders  for 
compressed  gas. 

When  the  carbon  is  from  0.6  to  0.85  per  cent.,  the 
steels  are  classed  as  high  carbon  or  hard  steels  and  among 
the  many  uses  may  be  mentioned  tires,  springs,  cheap 
cutlery,  wire,  certain  agricultural  tools  and  wood  work- 
ing tools.  Practically  no  steels  in  the  range  from  0.85 
to  1.25  per  cent,  carbon  are  used  without  some  heat 
treatment  which  will  modify  'the  pearlitic  character  to  a 
greater  or  less  degree. 

Rapidly  Cooled  and  Tempered  Steels. — The  changes 
which  take  place  when  a  steel  is  quickly  cooled  (quenched) 
and  subjected  to  a  later  heating  (tempering  or  annealing) 
are  so  numerous  that  only  a  few  can  be  considered  as 
illustrating  the  general  character  of  the  resulting  proper- 
ties. The  first  metallographic  constituent  of  chilled 
steel  which  might  be  expected  is  austenite  (Fig.  57) ,  but  it 
decomposes  so  quickly  on  cooling  that  it  is  never  formed 
in  the  commercial  hardening  of  plain  carbon  steel.  It  is 
the  chief  constituent  of  certain  alloy  steels,  however, 
and  will  be  considered  later  (p.  113).  The  constituent 
commonly  produced  when  steel  is  quenched  is  mar- 
tensite  (Fig.  58),  the  first  transition  product  of  the  de- 
composition of  austenite.  This  substance  is  extremely 
hard,  brittle  and  unworkable  so  that  pure  martensitic 
steel  is  rarely  found  in  practice.  Associated  with  other 


IRON  AND  STEEL  109 

constituents  such  as  troostite  or  free  ferrite,  in  low 
carbon  steels,  and  with  cementite,  in  the  high  carbon 
steels,  it  is  always  found  in  hardened  tool  steels  in 
amounts  which  vary  with  the  method  of  its  production. 
The  quenching  of  an  edged  tool  may  be  taken  as  a  single 
example  of  the  production  of  martensite.  The  tip  only 
of  the  tool  is  heated  to  redness  and  dipped  in  water. 
The  brittleness  of  the  resulting  martensite  is  reduced, 
with  a  consequent  sacrifice  of  hardness,  by  allowing 
heat  to  flow  from  the  unquenched  portion  of  the  tool 


FIG.  57. — Austenite    and    martensite.     (The    dark    masses    are    martensite 
and    the  light  ground  mass  austenite.)     350  X- 

to  the  tip,  until  the  desired  softening  results,  when  the 
entire  tool  is  quenched  and  the  final  product  has  the 
combined  properties  of  chilled  and  tempered;  steel. 

The  decomposition  which  takes  place  in  the  tool  just 
described,  after  it  has  been  subjected  to  partial  quench- 
ing, leads  to  the  formation  of  troostite  (Fig.  59).  As 
troostite  is  a  decomposition  product  of  martensite,  it  is 
to  be  expected  that  in  its  physical  properties  it  will  differ 
from  martensite  in  decreased  hardness  and  increased 
ductility.  Its  hardness  is  about  halfway  between  that  of 


110  PRINCIPLES  OF  METALLOGRAPHY 

martensitic  and  pearlitic  steel  of  the  same  carbon  content. 
The  tempering  of  troostite  leads  to  a  rapid  increase  in  duc- 


FIG.  58.— Martensite.     900 X. 


FIG.  59. — Martensite  and  troostite.     117X. 

tility  with  a  decrease  in  hardness.  It  may  be  formed 
by  cooling  slowly  through  the  transformation  interval, 
as  for  example  when  small  pieces  of  steel  are  quenched 


IRON  AND  STEEL  111 

in  oil,  but  it  is  much  more  frequently  produced  by  re- 
heating (tempering)  a  chilled  steel  below  400°.  It  is  a 
constituent  of  practically  all  plain  carbon  steels,  which 
have  been  hardened  (tools  for  example)  and  is  associated 
in  varying  amounts  with  martensite,  depending  on  the 
temperature  of  tempering.  When  great  hardness  is 
required  and  brittleness  is  of  less  importance,  as  in  the 
production  of  razor  blades  for  instance,  a  temperature  of 
about  200°  is  used,  producing  relatively  small  amounts  of 
troostite.  When  toughness  as  well  as  hardness  is  re- 


FIQ.  60. — Sorbite.     350  X. 

quired,  the  amount  of  troostite  is  increased  by  tempering 
at  300°  to  400°.  Most  tools  are  tempered  between  200° 
and  300°. 

While  it  is  almost  never  the  custom  in  practice  to 
temper  above  400°  it  is,  perhaps,  easier  to  correlate  the 
physical  properties  of  sorbite  (Fig.  60)  with  those  of 
troostite  by  considering  the  former  as  produced  by  the 
tempering  of  troostitic  steels  in  the  range  from  400°  to 
600°.  Sorbitic  steels  are  softer  and  more  ductile  than 
troostitic  and  not  as  soft  as  pearlitic  steels.  Sorbite, 
like  troostite,  is  one  of  the  decomposition  products  of 
austenite  and  is,  in  fact,  imperfectly  formed  pearlite. 


112  PRINCIPLES  OF  METALLOGRAPHY 

It  may  be  produced  by  heating  a  chilled  steel  in  the 
range  between  400°  and  650°  but  it  is  usually  made  by 
regulating  the  cooling  rate  in  such  a  way  that,  while 
the  chilling  action  is  not  great  enough  to  produce 
martensite,  it  is  too  rapid  to  allow  the  complete 
formation  of  pearlite.  It  may  be  formed  (1)  by  cool- 
ing small  pieces  in  air,  (2)  by  quenching  larger  pieces  in 
oil  from  a  temperature  just  above  the  critical  range  or  (3) 
by  quenching  small  pieces  in  water  from  a  point  near  the 
bottom  of  the  critical  range.  Though  slightly  less 
ductile  than  pearlitic,  sorbitic  steel  has  so  high  a  tensile 
strength  and  elastic  limit  that  it  is  used  for  the  highest 
grade  of  structural  work.  It  is  not  possible  to  give 
absolute  values  to  the  physical  properties  of  sorbite  as 
its  character  varies  so  much  with  the  method  of  pro- 
duction. It  may  be  stated  for  purposes  of  comparison 
that  while  sorbite  sometimes  reaches  a  tensile  strength 
of  140,000  pounds  per  square  inch,  ordinary  pearlite 
will  have  a  strength  of  about  110,000  pounds  per  square 
inch  while  the  coarsely  laminated  form  of  pearlite  will 
show  a  tensile  strength  of  only  about  70,000  pounds 
per  square  inch. 

The  following  table  from  H.  C.  Boynton  gives  the 
relative  hardness  of  the  different  constituents  of  steel  as 
compared  with  ferrite  (pure  iron)  as  a  standard.  It 
must  be  remembered  that  these  values,  particularly 
for  the  intermediate  forms  like  sorbite  and  troostite 
are  only  approximations. 

Ferrite         =      1 

Pearlite        =    43 

Sorbite         =    52 

Troostite     =    88 

Austenite     =104 

Martensite  =  239 

Cementite   =  272 


IRON  AND  STEEL  113 

Alloy  Steels. — In  addition  to  the  elements  silicon,  phos- 
phorus, manganese  and  sulphur,  always  found  in  small 
quantities  in  carbon  steels,  other  elements  are  often 
added  to  make  the  ternary  or  quaternary  alloy  steels, 
many  of  which  have  remarkable  physical  properties. 
The  subject  of  alloy  steels  is  so  large  and  the  informa- 
tion concerning  them  changing  so  rapidly  that  only  a 
very  general  discussion  can  be  given  here  in  spite  of  the 
great  and  constantly  increasing  importance  of  these 
alloys.  A  few  general  principles  seem  to  hold,  though 
even  these  must  not  be  accepted  as  absolutely  established 
but  rather  as  suggestive. 

1.  If  the  carbon  content  is  kept  constant,  the  addition 
of  the  alloying  element  in  increasing  amounts  causes  the 
steel  to  be  first  pearlitic,  then  martensitic  and  finally, 
with  a  sufficiently  high  percentage  of  the  third  element, 
to  become  austenitic. 

2.  By  holding  the  percentage  of  the  alloying  element 
constant  and  increasing  the  carbon  content  the  changes 
under  similar  cooling  conditions  are^  as  before,  from  pearl- 
itic, to  martensitic,  to  austenitic  steel. 

3.  It  follows,  almost  as  a  corollary  to  (1)  and  (2),  that 
the  higher  the  carbon  content  the  less  the  amount  of 
alloying   element    needed   to    complete   the   structural 
change  and,  conversely,  the  higher  the  percentage  of 
alloying  element  the  less  carbon  is  needed  to  change  the 
structure   and   properties.     These   changes   are   shown 
graphically  in  the  diagram  (Fig.  61). 

The  number  of  known  alloy  steels  is  great  and  con- 
stantly increasing.  Among  the  commoner  ternary  steels 
are  those  containing  either  nickel,  manganese,  tungsten, 
chromium,  vanadium,  molybdenum  or  silicon,  in  addi- 
tion to  the  carbon.  In  the  quaternary  class  may  be 
found  chrome-nickel,  chrome-tungsten,  chrome-vana- 
dium, nickel- vanadium  and  others.  Other  and  still  more 


114 


PRINCIPLES  OF  METALLOGRAPHY 


complex  series  are  the  chrome-nickel-vanadium,  chrome- 
tungsten-vanadium  and  the  like.  A  detailed  discussion 
of  these  alloys  is  impossible  and  only  a  few  will  be  con- 
sidered to  illustrate  in  a  general  way  the  properties 
of  each  group. 

The  strictly  metallic  elements  like  manganese,  nickel 
and  chromium  lower  the  critical  points  of  steel  very 
decidedly  so  that  austenite  and  martensite  can  be  formed 
much  more  easily  than  is  the  case  with  carbon  steel. 
A  steel  containing  from  1  to  1.5  per  cent,  carbon  and 


0.2          0.4         0.0          0.8         1.0 

Per  Cent  Carbon 
FIG.  61. — Constitutional   diagram   of    alloy   steels.     (Sauveur   after  Guillet.) 

from  10  to  15  per  cent,  manganese  can  be  obtained 
easily  in  the  austenitic  condition  by  reheating  the  cast 
steel  to  about  1000°C.  and  quenching  in  water  or  oil. 
The  resulting  steel  is  hard  and  resistant  to  wear  but, 
at  the  same  time,  possesses  much  ductility.  It  has 
been  used  in  making  rails  subjected  to  excessive  wear, 
as  on  sharp  curves. 

The  less  metallic  alloying  elements,  like  tungsten,  vana- 
dium and  molybdenum  have  little  or  no  effect  on  the 
critical  points  (p.  96)  but,  due  to  the  formation  of  double 
carbides,  tend  strongly  toward  the  production  of  cemen- 
titic  steels. 


IRON  AND  STEEL  115 

If  manganese  is  added  to  a  tungsten  steel,  the  alloy 
first  formed  on  cooling  is  of  the  cementite  class.  If, 
however,  this  steel  is  reheated  to  a  high  temperature 
and  cooled  in  the  air,  the  carbide  which  is  dissolved  at 
the  high  temperature  is  retained  in  the  martensitic  con- 
dition. Such  a  steel  is  said  to  be  "  self  hardening." 

One  of  the  most  important  of  the  alloy  steels  is  the 
chrome-tungsten  or  high  speed  tool  steel.  Such  an  alloy, 
with  from  10  to  20  per  cent,  tungsten  and  2  to  10  per 
cent,  chromium,  has  the  characteristics  of  cementite 
when  slowly  cooled.  On  reheating  to  a  very  high  tem- 
perature, often  almost  to  the  melting  point  of  the  steel, 
the  carbide  dissolves  and,  if  the  cooling  is  fairly  rapid, 
the  steel  retains  an  austenitic  or  martensitic  structure 
with  corresponding  physical  properties,  notably  great 
hardness.  The  striking  fact  in  this  case,  however,  is 
that  the  martensite  formed  in  this  way  shows  no  tendency 
to  soften  even  at  relatively  high  temperatures,  approach- 
ing 600°C.  This  makes  it  possible  to  run  a  cutting  tool 
at  such  speed  that,  while  its  edge  will  become  visibly 
red,  it  will  still  retain  its  hardness,  a  property  absolutely 
impossible  with  common  carbon  steel.  These  illustra- 
tions will  serve  to  show  some  of  the  possibilities  of  alloy 
steels. 

Heat  Treatment  of  Steel.1 — The  most  important 
property  of  steel  is  the  power  which  it  has  of  changing 
its  physical  condition  under  the  influence  of  heat. 
Many  of  the  changes  have  been  considered  in  connec- 
tion with  the  discussion  of  the  various  metallographic 
constituents:  sudden  cooling  or  quenching,  for  example, 
produces  the  hard  martensite;  tempering  hardened  steel 
softens  it,  producing  troostite  or  the  still  softer  con- 
stituent sorbite.  Annealing  may  be  carried  out  for 
one  of  three  reasons;  (1)  to  increase  the  softness  and 

1  BTJLLENS,  "Steel  and  Its  Heat  Treatment,"  Ed.  2. 


116  PRINCIPLES  OF  METALLOGRAPHY 

ductility,  (2)  to  relieve  the  strains  produced  by  chilling 
or  by  mechanical  work,  or  (3)  to  reduce  the  grain  size. 
The  second  object  has  been  considered  in  the  case  of 
worked  or  strained  brass  (p.  76)  to  which  strained 
steel  is  wholly  analogous.  Severely  worked  steel  shows 
the  same  elongated  structure  illustrated  in  Fig.  38 
and  the  restoration  of  its  normal  structure  by  annealing 
is  of  the  same  character  as  with  brass.  Improvement 
of  the  physical  properties  may  be  brought  about  by 
.annealing  whereby  the  size  of  the  crystal  grains  is 
reduced.  Large  crystal  grains  are  almost  always  an 
indication  of  weakness  while  the  production  of  small 
grains  invariably  leads  to  greatly  improved  mechanical 
properties. 

Temperature  of  Annealing. — The  steel  must  be  heated 
to  a  temperature  slightly  above  its  critical  range  in 
order  to  have  the  crystalline  structure  affected,  and  it 
has  been  found  that  the  higher  the  temperature  above 
the  critical  range  the  more  coarsely  crystalline  the  result- 
ing steel  will  be.  The  most  suitable  temperature  varies, 
of  course,  with  the  carbon  content  but  should  be  ap- 
proximately as  follows  for  plain  carbon  steels. 
(American  Society  for  Testing  Materials.) 

Carbon  content  Annealing  range 

Less  than  0. 12  per  cent 875°  to  925°C. 

0. 12  to  0.25  per  cent 840°  to  870°C. 

0.25  to  0.49  per  cent 815°  to  840°C. 

0.49  to  1.00  per  cent 790°  to  815°C. 

After  the  desired  temperature  has  been  reached  the  object 
must  be  kept  at  that  temperature  until  it  is  heated 
throughout  its  mass.  It  is  then  cooled  either  (1)  in 
the  annealing  furnace,  producing  the  softest,  weakest 
and  most  ductile  metal;  (2)  in  air,  giving  a  somewhat 
harder  and  less  ductile  material;  or  (3)  by  quenching  in 
oil,  giving  the  hardest,  strongest  and  least  ductile  steel  of 


IRON  AND  STEEL  117 

the  three.  The  finest  possible  structure  would  be  ob- 
tained by  quenching  from  a  point  as  near  the  critical 
range  as  possible,  but,  except  with  very  low  carbon  con- 
tent, the  resulting  steel  would  be  hard  and  lacking  in 
ductility.  The  double  anneal  overcomes  this  difficulty. 
The  operation  consists  in  reheating  the  hardened  steel 
to  about  650°  (close  to,  but  below  its  critical  range), 
which  serves  to  relieve  the  hardness  and  at  the  same 
time  to  retain  the  fine  structure. 

Case  Hardening. — An  operation  closely  allied  to  heat 
treatment  is  case  hardening,  a  process  which  is  carried 
on  for  the  purpose  of  adding  a  hard,  non-abrasive  sur- 
face to  a  strong,  ductile  steel.  The  steel  used  for  this 
purpose  has  a  low  carbon  content  (0.2  per  cent,  or  less) 
and  is  carburized  by  heating  in  contact  with  a  carbon 
furnishing  substance,  either  in  the  solid,  liquid  or  gase- 
ous form.  Numerous  materials  have  been  used  of  which 
charred  leather,  potassium  ferrocyanide  and  barium  car- 
bonate may  be  considered  types.  As  carbon  dissolves 
very  slightly  in  a-iron,  if  at  all,  the  case  hardening  tem- 
perature must  be  above  the  critical  range  and  is  usually 
between  850°  and  1000°C.  The  depth  to  which  the 
carbon  penetrates  depends  on  the  length  of  time  during 
which  the  steel  is  in  contact  with  the  carbonaceous 
material  and  varies  from  0.5  millimeter  to  5  millimeters 
with  an  average  depth  of  from  2  to  3  millimeters. 
Metallographic  examination  of  a  case-hardened  specimen 
shows  a  surface  coat  of  hard  cementite  over  a  band  of 
eutectoid,  free  from  cementite  and  ferrite,  and  below 
this  band  the  soft  interior  mass  or  core  in  which  ferrite 
largely  predominates.  The  coarse  structure  of  the  core 
produced  by  the  long  heating  during  the  case  hardening 
process,  must  then  be  refined  by  suitable  heat  treatment. 

Cementation  is  a  similar  operation  applied  to  wrought 
iron  for  the  purpose  of  changing  it  into  steel.  It  differs 


118  PRINCIPLES  OF  METALLOGRAPHY 

from  case  hardening  in  that  the  steel  produced  in  this 
way  is  afterward  melted  and  the  carbon  becomes 
uniformly  distributed  in  the  ingot,  forming  cement  steel. 

Cast  Iron. — Reference  to  the  iron-carbon  diagram 
(p.  99)  shows  that  the  cast  iron  field  extends  from  1.7 
to  4  or  5  per  cent.  C.  The  liquidus  curve  has  two 
branches,  along  one  of  which  (FeE)  austenite  should 
separate  and  along  the  other  (CE)  cementite.  The 
cementite  separating  along  CE  is  so  readily  decomposed 
on  cooling  that  three  classes  of  cast  irons  are  commonly 
recognized,  depending  on  the  cooling  rate  and  on  the 
presence  of  constituents  other  than  carbon. 

1.  White  iron  results  from  rapid  cooling  (chill  cast- 
ing) of  the  metal  and  has  the  white  fracture  and 


FIG.  62. — Chilled  casting.     (Actual  size.)     a  layer  is  white  iron. 

hard,  brittle  qualities  of  cementite.  Chill  castings 
are  too  hard  to  machine  and  are  seldom  used  in  the 
production  of  small  articles.  It  is  often  necessary  to 
produce  soft,  fairly  strong  cores  with  extremely  hard 
surfaces  as,  for  example,  in  car  wheel  treads  or  the  sur- 
faces of  rolls.  In  such  a  case  the  white  iron  (cementite) 


IRON  AND  STEEL 


119 


surface  may  be  produced  by  casting  against  a  highly 
heat-conducting  material  like  an  iron  plate.     The  pro- 


FIG.  63. — Section  made  from  white  border,  a,  in  Fig.  62.     350  X. 


FIG.  64. — Graphic  temper   carbon  in  malleable  iron.     350  X. 

duction  of  white  iron  is  also  favored  by  the  absence,  or 
low  percentage,  of  silicon  and  the  presence  of  high 
percentages  of  manganese  and  sulphur  (Figs.  62  and  63). 


120  PRINCIPLES  OF  METALLOGRAPHY 

An  important  decomposition  product  of  white  iron  is 
produced  by  annealing  for  several  days  at  a  temperature 
of  about  730°C.  Under  these  conditions  the  hard,  white 
cementite  decomposes  into  graphite  and  ferrite,  the 
graphite  separating,  however,  not  in  the  massive  form 
but  as  an  amorphous  black  powder  (temper  carbon) 
(Fig.  64).  This  malleabilizing  process  produces  an  iron 
which  is  much  stronger  than  gray  iron  of  the  same  com- 


FIG.  65A. — Taken  at  the  part  y,  in  Fig.  62.     350  X  shows  gray  iron. 

position.  The  tensile  strength  of  malleable  iron  is 
more  than  40,000  pounds  per  square  inch  as  against  an 
approximate  20,000  pounds  for  gray  iron  with  the  same 
amount  of  free  carbon  in  the  fibrous  instead  of  the 
powdery  form.  Malleable  iron  is  used  for  castings  which 
are  to  be  subjected  to  shock,  especially  in  the  manufac- 
ture of  small  castings  which  would  be  made  of  steel  if 
it  were  not  for  the  technical  difficulties  involved  in  steel 
casting. 

2.  Gray  cast  iron  is  produced  when  the  cementite 
first  formed  is  allowed,  by  decreasing  the  rate  of  cooling, 
to  decompose  into  ferrite  and  graphite.  This  gives  a  dull 


IRON  AND  STEEL  121 


FIG.  65B. — Graphite  in  iron.     115X-     The  halves  represent  two  specimens  of 
*  iron  with  different  carbon  content. 


FIG.  66. — Section  taken  at  the  transition  zone,  /3  in  Fig.  62  mottled  iron 
350  X. 


122  PRINCIPLES  OF  METALLOGRAPHY 

gray  appearance  to  the  fractured  metal.  Under  these 
conditions,  the  graphite  separates  in  the  form  of  long 
fibrous  masses  unaffected  by  etching  reagents  (Figs. 
65a  and  656).  The  separation  of  graphite  is  increased  by 
the  addition  of  silicon  which,  in  gray  castings,  is  usually 
present  in  amounts  varying  from  2  to  4  per  cent.  The 
tensile  strength  of  gray  iron  varies  from  18,000  to  about 
23,000  pounds  per  square  inch. 

3.  Mottled  Iron. — By  a  suitable  regulation  of  the 
cooling  rate,  an  iron  containing  both  free  cementite  and 
graphite  is  produced  with  properties  depending  on  the 
relative  amounts  of  the  soft  and  hard  constituent 
(Fig.  66). 


CHAPTER  VI 
DEFECTIVE  MATERIAL 

Not  the  least  important  of  the  many  uses  of  metallog- 
raphy is  its  application  to  the  study  of  defective  or 
unsuitable  material.  A  distinction  must  be  made  be- 
tween the  terms  "defective"  and  " unsuitable"  as  a 
perfect  alloy  may  be  wholly  unsuited  to  the  purpose  for 
which  it  has  been,  or  is  to  be,  used.  The  causes  of 
defective  metal  are  numerous  but  they  may  generally  be 
classified  in  one  of  four  groups: 

1.  Incorrect  chemical  composition; 

2.  Improper  mixing,  melting  or  casting; 

3.  Unsuitable  mechanical  treatment; 

4.  Improper  heat  treatment. 

1.  Incorrect  Chemical  Composition. — Large  errors  in 
chemical  composition  produce  effects  which  are  too  ob- 
vious to  need  more  than  a  reference.  If  an  alloy  which 
was  supposed  to  be  brass  with  70  per  cent,  copper, 
actually  contained  but  50  per  cent,  copper  and  was  sub- 
jected to  severe  cold  work  the  results  would  be  disas- 
trous. Such  errors,  however,  are  found  by  the  analyst 
rather  than  by  the  metallographist. 

Slight  errors  in  chemical  composition  are  generally 
far  more  serious  because  they  are  so  much  less  readily 
detected.  Not  infrequently  these  differences  in  chem- 
ical composition  are  so  localized  as  to  escape  detection 
by  analytical  processes  and  yet  they  are  of  far-reaching 
effect  on  the  physical  properties  of  the  material.  Some 
of  these  defects  have  been  referred  to,  as  for  instance, 

123 


124  PRINCIPLES  OF  METALLOGRAPHY 

the  formation  of  brittle  envelopes  around  brass  crystals 
when  bismuth  or  antimony  is  present  (p.  92).  The 
commonest  chemical  defect  is  the  presence  of  segregated 
material  which  is  without  question  one  of  the  most 
common  causes  of  failure  in  metals.  Segregated  im- 
purities occur  in  many  technical  alloys  but  to  a  far 
greater  extent  in  steel  and  iron  in  which  their  presence 
may  do  serious  harm.  Until  recently  but  little  infor- 


FIG.  67. — Slag  inclusions  in  wrought  iron.     350  X. 

mation  has  been  available  on  defects  of  this  sort  in  non- 
ferrous  metals.  During  the  past  year,  however,  two 
papers1  have  been  published  showing  by  microphoto- 
graphs,  inclusions  of  the  oxides  of  tin  and  zinc  as  well  as 
of  casting  sand  and  other  non-metallic  substances  in  brass 
and  bronze. 

In  the  case  of  iron  and  steel,  chemical  inclusions  usu- 

1CoMSTocK,  "Non-metallic  Inclusions  in  Brass  and  Bronze,"  /.  Am. 
Inst.  Metals,  March,  1918.  CARPENTER  &  ELAM,  "An  Investigation 
on  Unsound  Castings  of  Admiralty  Bronze,"  J.  Inst.  Metals  (British), 
xix,  155. 


DEFECT  I VE  MA  TERIAL 


125 


ally  consist  of  slag  (commonly  silicates  of  iron  or  other 
elements),  sulphides  of  iron  or  manganese,  phosphides 
of  iron  or  manganese  or  metallic  oxides.  Slag  is  a 
normal  constituent  of  wrought  iron  but  may  at  times 
segregate  in  such  a  way  as  to  become  harmful  (Fig.  67). 
Small  amounts  of  oxide  and  silicate  are  always  present 
in  steel  and  if  the  quantity  is  not  excessive  and  is  uni- 


FIG.   68. — Islands    of    manganese    sulphid 


formly  distributed  will  not  seriously  effect  the  mechan- 
ical properties  of  the  metal.  Irregular  distribution  of 
the  non-metallic  material  in  segregated  areas  is  always 
a  source  of  danger  and  can  be  detected  more  readily 
by  the  metal  microscope  than  in  any  other  way. 

Sulphides,  either  of  manganese  or  iron,  are  sources 
of  danger,  especially  in  material  which  is  to  be  worked 
hot  (rolled,  hammered  or  drawn).  Manganese  sul- 
phide is  sometimes  found  in  steel  rails  in  a  form  which 


126 


PRINCIPLES  OF  METALLOGRAPHY 


under  the  microscope  resembles  long,  narrow  islands, 
dove  gray  in  color  (Fig.  68) .  Sulphide  inclusions  are 
easily  detected  by  the  production  of  "sulphur  prints." 
In  the  older  method  of  Heyn  this  is  effected  by  hold- 


FIG.  69. — Sulphur  print  on 


(Actual  size.) 


ing  a  piece  of  white  silk  moistened  with  mercuric  chlo- 
ride and  hydrochloric  acid  in  contact  with  the  polished 
surface  of  the  metal  (Fig.  69).  Black  stains  of  mercuric 
sulphide  are  produced  at  the  points  of  contact  with  the 


FIG.  70. — Sulphur  print  of  a  defective  boiler  plate  showing  sulphide  streaks. 

sulphide  spots.  The  same  effect  is  obtained  more 
simply  by  soaking  photographic  printing  paper  (Velox 
or  Cyko)  in  2  per  cent,  sulphuric  acid  and  applying  the 
paper  to  the  polished  sample.  Black  spots  or  streaks 


DEFECTIVE  MATERIAL  127 

of  silver  sulphide  are  produced.  The  print  may  be 
made  permanent  by  fixing  it  in  "hypo"  (sodium  thio- 
sulphate)  solution  in  the  usual  way.  The  presence  of 
sulphide  streaks  is  shown  in  the  following  photograph, 


A.    Rolled    from    the    top    of    the         B.  Rolled  from  the  bottom  of  the 
ingot.  ingot. 

FIG.  71. — Steel    I-beams    showing    sulphide    and    phosphide    segregations. 

Fig.  70.  That  the  lines  are  actually  due  to  sulphide 
inclusions  is  shown  by  chemical  analysis  of  the  separate 
layers  (Heyn).  Layer  I  shows  0.067  per  cent,  sulphur, 
layer  II,  0.201  per  cent,  and  layer  III,  0.240  per  cent. 


128  PRINCIPLES  OF  METALLOGRAPHY 

Phosphorus  in  the  form  of  phosphide  is  one  of  the  most 
harmful  of  the  non-metallic  inclusions  in  mild  steel. 
It  occurs  in  the  ingot  and  if  too  small  an  amount  of  the 
ingot  top  is  removed  to  eliminate  the  phosphide  segre- 
gations, these  will  go  through  the  mechanical  operations 
of  rolling,  forging,  etc.,  without  coming  to  the  surface. 
The  result  is  shown  in  Fig.  71  and  illustrates  in  a  striking 
fashion  the  advantages  of  metallographic  over  chemical 
examination  in  cases  of  this  sort.  The  streaks  are  due 
in  this  case  to  sulphide  as  well  as  to  phosphide  inclu- 
sions. Phosphide  inclusions  are  the  usual  cause  of  "cold 
shortness." 

This  inclusion  of  non-metallic  impurities  in  alloys 
has  been  estimated  to  cause  more  than  75  per  cent,  of 
the  failures  found  in  technical  practice. 

2.  Improper  Melting,  Mixing  or  Casting. — Another, 
though  less  frequent,  reason  for  defective  material  is 
imperfect  mixing  of  the  molten  alloy,  producing  layer 
formation  in  the  solid.  Plastic  bronzes  (p.  9)  some- 
times fail  because  of  imperfect  mixing  which  causes  a 
much  greater  concentration  of  lead  in  one  part  of  the 
casting  than  in  another.  In  the  light  bearing-alloys 
of  the  Babbitt  metal  class  and  in  type  metals  containing 
antimony,  irregular  distribution  of  the  cubical  crystals 
of  SbSn  is  of  common  occurrence  and  is  always  a  cause  of 
unsatisfactory  material  (Fig.  72  a  and  b).  Segregation  in 
brasses  of  the  Muntz  metal  type  is  not  at  all  infrequent 
and  in  rolled  or  drawn  material  gives  rise  to  distinct 
layer  formation.  Fig.  73  shows  a-  and  0-brass  in 
adjacent  streaks  in  a  condenser  tube.  The  a-brass 
may  be  found  also  in  the  form  of  islands  or  spots  in  the 
(3-field  as  shown  in  Fig.  74  a  and  6.  These  few  cases 
illustrate  the  possibilities  of  layer  formation  in  alloys 
which  are  duplex  in  structure  and  have  components 
differing  in  specific  gravities. 


DEFECTIVE  MATERIAL 


129 


3.  Unsuitable  Mechanical  Treatment. — Work,  either 
cold  or  hot,  may  produce  bad  material  if  improperly  done. 


FIG.  72. — Variations  in  size  and  distribution  of  SbSn  crystals  in  babbitt 
metal. 

The  effects  of  cold  work  on  brass  have  been  considered 
on  p.  72  and  illustrated  in  Figs.  18  and  47.     The  char- 


130 


PRINCIPLES  OF  METALLOGRAPHY 


acteristic  fibrous  structure  of  overworked  brass  is  also 
shown  in  Fig.  75.  Steel  is  more  commonly  worked  hot 
and  defective  metal  may  be  produced  when  the  rolling, 
hammering  or  other  operation  is  commenced  at  too 
high  a  temperature.  If,  for  example,  a  steel  containing 
ferrous  sulphide  (FeS)  is  rolled  at  a  temperature  much 
above  900°C.  it  is  probable  that  the  iron  sulphide  is 
actually  molten  and  rolling  would  naturally  cause  serious 


FIG.  73. — Layer  formation  in  defective  condenser  tube  showing  a-  and  a  + 
/3-layers.     75  X. 

cracks  in  the  finished  material.  If,  on  the  other  hand, 
mechanical  operations  are  carried  on  (or  are  finished) 
at  too  low  a  temperature,  severe  strain  hardening  may 
result.  Both  these  conditions,  incipient  cracking  from 
too  hot  work  and  overstrain  from  cold  work  may  be 
detected  by  the  microscope.  Unequal  amounts  of  work 
on  different  parts  of  the  same  article  is  a  frequent  cause 
for  metal  failure.  This  produces  internal  strains  which 
may  ultimately  lead  to  " season  cracks"  (p.  91)  or  may 


DEFECTIVE  MATERIAL 


131 


cause  immediate  cracking  on  reannealing.     Fig.  18,  p.  37, 
shows  a  crack  which  was  probably  produced  in  this  way. 


A.  Etched 


,-ith    NH4OH    and    (NH^tStOs.     a-brass   very   slightly 
attached.     75 X.  , 


B.  Part  of  the  same  tube  etched  with  NH4OH  and  H2O2.     75  X- 
FIG.  74. — Formation  of  islands  of  a-brass  in  |8  field. 

Cold  work  may  be  done  unintentionally  in  certain 
mechanical  operations  and  may  cause  serious  trouble. 


132 


PRINCIPLES  OF  METALLOGRAPHY 


In  fitting  large  pieces  of  structural  steel  such  as  are  used 
in  bridges,  building  frames  and  the  like,  it  is  sometimes 
necessary  to  hammer  the  parts  into  position.  This 
has  been  known  to  cause  bad  strain  hardness  in  a  limited 
area  with  a  resulting  failure  of  the  metal.  In  a  gas  tank 
recently  constructed  a  large  number  of  rivets  broke 
because  of  the  overstrain  produced  by  hammering  when 
they  were  too  cold.  Defects  produced  in  this  way  can- 
not, of  course,  be  controlled  by  metallographic  exami- 


FIG.  75. — or-brass  strained  and  not  annealed.     75  X. 

nation  but  the  causes  of  failure  can  often  be  detected 
by  the  microscope  and  the  responsibility  placed. 

One  other  type  of  defective  material  is  due  to  a  combi- 
nation of  mechanical  and  chemical  difficulties  and  is 
caused  by  the  rolling  or  hammering  of  surface  oxides  or 
" scale"  into  the  metal  under  treatment.  This  is  one 
of  the  reasons  for  hard  spots  in  rolled  brass  and,  as  it 
is  apt  to  be  a  surface  condition,  it  frequently  leads  to 
rapid  corrosion  of  the  material. 

4.  Improper  Heat  Treatment. — The  term  "heat  treat- 
ment," is  most  commonly  used  in  connection  with  heating 


DEFECTIVE  MATERIAL  133 

or  cooling  operations  (quenching,  tempering,  etc.)  ap- 
plied to  the  solid  alloys  but,  if  the  term  is  applied  in  a 
broader  sense  to  include  the  rate  at  which  a  molten  alloy 
cools,  it  explains  many  cases  pf  defective  material.  The 
casting  of  Babbitt  metal  will  serve  to  illustrate  the  way 
in  which  defective,  or  at  least  unsuitable,  material  may 
be  produced  by  variations  in  the  cooling  rate  of  the 
liquid  metal.  Too  rapid  cooling  produces  undeveloped 
crystals  of  the  tin-antimony  compound  SnSb,  while 
very  slow  cooling  causes  the  formation  of  large  cubes 
of  the  compound  and  greatly  increases  the  tendency 
of  these  crystals  to  rise  to  the  top  of  the  cast  metal. 
Both  conditions  are  responsible  for  unsatisfactory  bearing 
surfaces.  If  the  metal  against  which  the  Babbitt  is 
poured  has  a  temperature  of  approximately  100°C. 
the  SnSb  crystals  will  be  found  to  be  moderate  in  size 
and  regularly  distributed. 

Heat  treatment,  as  applied  to  solid  alloys,  consists  of 
annealing,  tempering  or  quenching  and  any  one  of  these 
operations  if  badly  executed  will  lead  to  defective  or  un- 
suitable material.  The  microscope  is  an  invaluable 
tool  in  studying  defects  of  this  kind.  Annealing  carried 
on  at  too  high  a  temperature  may  cause  " burning"  or 
" overheating."  Burning  is  nothing  but  oxidation  and 
is  visible  under  the  microscope  as  a  marked  thickening 
of  the  grain  boundaries  due  to  oxide  formation.  It  is 
a  cause  of  brittleness  and  produces  fracture  between 
the  grains  because  of  the  oxide  envelope  with  which  the 
crystals  are  surrounded.  Overheating  is  distinguished 
from  burning  in  that  it  is  not  accompanied  by  oxidation 
but  is  a  cause  of  excessive  crystal  growth.  The  very 
large  crystals  of  overheated  a-brass  are  shown  in  Fig. 
50  (p.  89).  In  the  case  of  steel  the  best  physical 
properties  are  associated  with  fine  structure  so  that 
annealing  at  temperatures  too  far  above  the  critical  range 


134  PRINCIPLES  OF  METALLOGRAPHY 

will  produce  material  which,  if  it  is  not  actually  defective, 
is  at  least  inferior  to  the  steel  which  would  be  obtained 
by  suitable  treatment. 

Annealing  in  an  unsuitable  atmosphere  may  also  pro- 
duce defective  material.  Steel,  for  instance,  may  be 
decarbonized  and  therefore  materially  weakened  by 
long  heating  in  an  oxidizing  atmosphere. 

Tempering  is  carried  out  for  the  purpose  of  relieving 
strain  hardness  or  increasing  the  amount  of  softer  ma- 
terial at  the  expense  of  the  hard  material  as,  for  example, 
in  the  change  of  martensite  to  troostite  or  sorbite.  Tem- 
pering at  too  high  a  temperature  may  unduly  decrease 
the  hardness  and  at  too  low  a  temperature  may  leave  an 
excessive  amount  of  the  harder  constituent.  In  either 
case  the  resulting  material  is  defective  in  the  sense  that 
it  is  not  in  the  proper  mechanical  condition  for  the  pur- 
pose to  which  it  is  to  be  applied. 

Quenching  incorrectly  performed  is  another  cause  of 
defective  or  unsuitable  material.  Quenching  from  too 
high  a  temperature  generally  forms  excessively  large 
crystals  which  produce  correspondingly  weak  metal  and 
it  may  also  cause  a  form  of  strain  hardness  which  not 
infrequently  leads  to  cracks.  Quenching  is  done  for  the 
purpose  of  retaining  the  metal  in  the  condition  (usually 
a  hard  state)  in  which  it  exists  at  the  higher  temperature. 
If  the  temperature  from  which  the  metal  is  quenched  is 
too  low  the  desired  hardness  will  not  be  obtained  and  the 
structure  of  the  soft  form  will  be  seen  under  the  micro- 
scope. Finally,  if  the  specimen  is  large,  the  cooling 
rate  may  be  unequal  in  different  parts  and  local  differ- 
ences in  hardness  and  strain  will  be  produced.  Warp- 
ing or  cracking  of  the  quenched  piece  not  infrequently 
results. 

This  brief  discussion  of  some  of  the  causes  of  defective 
material  will  serve  to  illustrate  the  applications^  of 


DEFECTIVE  MATERIAL  135 

metallography  to  an  important  engineering  problem 
and  also  emphasize  the  value  of  metallographic  control 
both  to  the  producer  and  to  the  consumer  of  alloys. 
This  new  branch  of  physical  science  is  only  one  of  the 
factors  in  the  study  of  metals  and,  if  possible,  should 
always  be  used  in  connection  with  ordinary  chemical 
analysis  and  mechanical  testing  rather  than  indepen- 
dently of  them.  There  are  times,  however,  when  it  is 
the  only  means  of  study  that  may  be  used  without 
damage  to  the  material  under  investigation  as,  for 
instance,  in  the  inspection  of  a  finished  article. 

In  conclusion  it  must  be  said  that,  while  metallog- 
raphy has  become  an  almost  indispensable  science  in  the 
metal  industries,  the  interpretation  of  microscopic  ap- 
pearances in  complex  cases  requires  a  highly  skilled 
and  experienced  observer.  The  inexperienced  metallog- 
raphist  should  be  extremely  careful  not  to  overestimate 
the  value  of  his  observations. 


APPENDIX 

TABLE  1 

Outline  of  a  Brief  Course  in  Experimental  Metallography 

1.  Assemble  a  melting  point  apparatus  and  calibrate  a  thermo- 
element (p.  18). 

2.  Prepare  a  series  of  alloys,  study  the  cooling  curves  and  construct 
the    equilibrium    diagram*    (p.    4).     Lead-antimony,    bismuth-tin 
and  lead-tin  are  suggested. 

3.  Sectionalize,  polish,  etch  and  examine  the  alloys  prepared  in 
(2)   (p.  29). 

4.  Photograph     at    a    magnification    of    seventy-five  diameters 
(75 X),  develop  the  plates,  print,  trim  and  mount  the  photographs 
(p.  34). 

5.  If  time  permits  study  as  in  (2),  (3)  and  (4)  the  following  alloys: 
(a)  Tin-magnesium. — (Open  maximum.) 

(6)  Copper-antimony. — (Open  maximum  and  concealed  maxi- 
mum.) 

(c)  Tin-antimony. — (Several  series  of  solid  solutions,  probably 
with  a  compound.) 

6.  Experiments  with  Brass. — Polish,  etch  and  examine  (p.  29). 
(a)  Cast  brass. 

(6)  Cold-worked  a-brass. — (Examples  of  this  material  occur  in 
spring  brass,  sheets,  condenser  tubes,  cartridge  cases  and 
similar  articles.) 

(c)  Examine    severely  cold-worked   a-brass.     Anneal   for   ten 
minutes  at  550°C.  and  reexamine. 

(d)  Anneal  samples  of  cold-worked  a-brass  for  ten  minutes  each 
at  the  temperatures  450°,  550°,  650°,  750°  and  850°  and  com- 
pare the  resulting  crystals  with  reference  to  size. 

(e)  Examine  cast  Muntz  metal  (60  per  cent.  Cu-40  per  cent.  Zn). 
(/)  Examine  extruded  or  hot-worked  Muntz  metal. 

(g)  Anneal  several  specimens  of  hot-worked  Muntz  metal  at 
800°  and 

*  If  but  a  limited  time  is  available,  the  number  of  mixtures  may  be 
divided  among  the  members  of  the  class  so  that  each  will  make  but  one  or 
two  melts.  The  results  are  then  collected  and  arranged  in  the  form  of  the 
complete  diagram. 

136 


APPENDIX  137 

(1)  Quench  in  cold  water  (/3-brass). 

(2)  Cool  in  air. 

(3)  Cool  slowly  in  the  annealing  furnace.     Anneal  a  quenched 
specimen  one-half  hour  at  400°  and  compare  with  (1). 

7.  Examination  of  Bronze. — (a)  Examine  cast  bronze  and  compare 
its  structure  with  manganese  bronze  and  aluminum  bronze. 

(6)  Examine  cold-worked  bronze  and  anneal  to  restore  normal 

structure, 
(c)  Examine  leaded  phosphor-bronze  after  polishing  and  before 

and  after  etching.     (The  unetched  specimen  will  show  the 

distribution  of  lead.) 

8.  Experiments   with  Steel. — (a)  Examine  cast  steel  with  about 
0.3  per  cent,  carbon.    Anneal  for  one  hour  at  875°  and  compare 
grain  size  with  that  of  the  original  sample.     (Etch  with  HNOs, 
p.  103.) 

(&)  Heat  specimens  of  steel  containing  approximately  0.1  per  cent., 
0.3  per  cent.,  0.6  per  cent,  and  0.9  per  cent.  C  and  cool  very 
slowly  through  the  critical  range  or  heat  for  30  minutes  just  be- 
low the  critical  range.  (Note  the  relative  quantities  of  ferrite 
and  pearlite.)  Examine  at  100  X  and  500  X. 

(c)  Quench  a  small  specimen  with  about  1.2  per  cent.  C  from 
1200°    (Martensite). 

(d)  Quench  a  small  specimen  with  about  0.8  per  cent.  C  from 
1000°  and  reheat  to  600°  (Sorbite). 

(e)  Quench  steel  used  in  (d}  as  before  and  reheat  to  350°  (Troostite) . 
Examine  at  100  X  and  500  X. 

(/)  Polish  two  specimens  of  normally  cooled  high  carbon  steel 
(about  1.4  per  cent.  C).  Etch  one  piece  with  nitric  acid,  the 
other  with  sodium  picrate  (p.  104).  Examine  at  100  X  and 
500  X  (Cementite). 

(g)  Quench  a  specimen  with  at  least  1  per  cent,  manganese 
arid  about  1.5  per  cent,  carbon  from  1400°  in  ice  water 
(Austenite) . 

(h)  Examine  high-speed  tool  steel.  Heat  for  twenty  minutes 
between  500°  and  600°  and  reexamine  after  slow  cooling. 

(i)  Heat  a  sample  of  steel  containing  less  than  0.2  per  cent,  car- 
bon in  a  mixture  of  60  per  cent,  charcoal  and  40  per  cent,  barium 
carbonate  for  at  least  two  hours  at  1000°.  Cool  and  examine 
the  entire  cross  section  (case  hardening).  Note  the  thickness 
of  the  cementite  layer. 

0')  Select  two  pieces  of  steel  with  0.5  per  cent.  C.     Heat  one  to 


138  PRINCIPLES  OF  METALLOGRAPHY 

900°,  the  other  to  1200°.     Cool  both  in  air  and  examine  with 
special  reference  to  size  of  the  crystal  grains. 
Experiments   with   Iron. — (a)   Wrought  Iron. — Polish,  etch  with 
alcoholic  HN03  and  examine  wrought  iron  using  one  longitudinal 
and  one  transverse  section.     Note  especially  the  color  and  distribu- 
tion of, the  slag. 

(6)  Gray  Cast  Iron. — Polish  a  high-silicon,  high-carbon  iron  and 
examine  without  etching  (Graphite). 

(c)  White  Cast  Iron. — Examine  a  high -manganese,  low-silicon  iron 
which  has  been  chill  cast. 

(d)  Malkable  Iron. — Examine  malleable  iron  etched  and  unetched. 
Note  the  small  rounded  masses  of  graphite  due  to  the  decomposition 
of  cementite  and  compare  these  with  the  graphite  plates  or  needles 
in  (6). 

(e)  Mottled  Iron. — Examine  the  transition  stage  which  will  be  found 
between  the  outer  case  and  inner  core  of  a  chilled  casting. 

Mechanical  Testing. — -If  testing  machines  are  available  a  study 
of  the  various  mechanical  properties  as  hardness,  tensile  strength, 
elastic  limit,  elongation,  etc.  should  be  made  with  some  of  the  com- 
moner industrial  alloys,  brass,  the  different  bronzes,  steel,  iron  and  the 
like  and  the  results  obtained  studied  in  connection  with  the  metallo- 
graphic  examination. 

The  outline  just  given  is  merely  suggestive  and  the  number  of 
experiments  may  well  be  increased  by  the  study  of  defective  material 
as,  for  example,  broken  rails  or  axles,  corroded  boiler  or  condenser 
tubes,  overheated  steel  or  any  of  the  cases  of  the  same  sort  that  may 
especially  interest  the  experimenter.  The  field  for  investigation 
and  research  is  unlimited. 


APPENDIX  139 


TABLE  2 

Books  and  Journals. — (This  list  is  not  complete  but  includes  the 
publications  most  readily  obtainable).  The  asterisk  *  indicates  the 
books  especially  recommended  for  the  general  reader. 

General 

Alliages  Me"talliques — Cavalier — 1909. — General  metallography 
dealing  extensively  with  physical  and  mechanical  properties. 

Die  binaren  Metallegierungen — Bornemann — 1909. — Brief  theoretical 
discussion  and  many  equilibrium  diagrams. 

Einfiihrung  in  die  Metallographie — Goerens — 1906,  *  Translation — 
Introduction  to  Metallography — Ibbotson — 1908. — A  small 
general  metallography  but  a  classic  in  that  it  is  the  first  book 
to  deal  with  equilibrium  diagrams  as  well  as  microphotography. 

Etude  Industrielle  des  Alliages  Me"talliques — Guillet — 1906 — Gen- 
eral study  of  industrial  alloys  with  many  microphotographs. 

Etude  Theorique  des  Alliages  "Me'talliques— Guillet— 1904— Dis- 
cussion of  the  methods  of  alloy  study;  hardness,  conductivity, 
magnetism,  density,  etc. 

Handbuch  der  Metallographie — Guertler— 1912-1914. — The  most 
complete  and  detailed  treatment  of  metallography  yet  published. 
Much  emphasis  is  laid  on  theoretical  considerations. 

*  Introduction  to  Physical  Metallurgy — Rosenhain — 1917. — Deals 

especially  with  mechanical  testing,  physical  properties  and  indus- 
trial applications  of  alloys.  Summary  of  defective  material. 
Lehrbuch  der  Metallographie — Tammann — 1914. — Deals  with  the 
subject  almost  wholly  from  the  theoretical  viewpoint.  Contains 
an  extended  discussion  of  the  valence  theory  as  related  to 
intermetallic  compounds. 

*  Metallic  Alloys— Gulliver— 1913. — One  of  the  standard  books  on 

general  metallography.  Considers  crystallography  and  the 
mechanism  of  crystallization  of  metals.  Excellent  treatment 
of  ternary  alloys. 

Metallographie — Heyn  and  Bauer — 1909. — Small  general  metallo- 
graphy largely  theoretical. 

Metallographie — Ruer — 1907  (Translated  by  Mathewson)  .—Ele- 
ments of  metallography  considered  chiefly  from  the  theoretical 
side. 

*  Metallography — Desch — 1913. — General  metallography  with  spe- 

cial emphasis  on  physico-chemical  principles  and  an  extended 


140  PRINCIPLES  OF  METALLOGRAPHY 

discussion  of  the  connection  between  constitution  and  physical 

properties. 
Metallography — Hiorns — 1902 — An  introduction  to  the  microscopic 

study  of  alloys. 
Physikalische  Chemie  der  Metalle— Schenk— 1909.— Six  lectures  on 

applications  of  physical  chemistry  to  metallography. 
Traite"  de  Metallographie — Robin — 1912. — A  large  work  on  general 

metallography  including  many  subjects  not  usually  considered. 

A  complete  set  of  diagrams  and  many  microphotographs. 
Traite   Complet   D'Analyse  Chimique — Goerens  and  Robin — 1911 

Metallographie    Microscopique. — General  metallography  with 

many  diagrams  and  microphotographs. 

IntermetaJic  Compounds 

Chemical  Combinations  Among  Metals — Giua — 1918  (Translated 
by  Robinson). — Theoretical  treatment  of  the  nature  and 
properties  of  intermetallic  compounds. 

Intermetallic  Compounds — Desch — 1914. — A  monograph  dealing 
with  the  constitution  and  physical  properties. (hardness,  conduc- 
tivity, etc.)  of  intermetallic  compounds. 

Industrial  Alloys  (Non-ferrous) 

Alloys — Sexton — 1909. — Deals  with  the  physical  and  mechanical 
properties  of  the  non-ferrous  alloys  and  the  methods  of 
preparation. 

Alloys  and  Their  Industrial  Applications — Law — 1914. — A  book 
designed  primarily  for  the  engineer  with  a  brief  theoretical 
discussion  and  a  detailed  description  of  the  mechanical  proper- 
ties of  technical  alloys. 

Mixed  Metals — Hiorns — 1912. — A  descriptive  text-book  on  the 
composition,  methods  of  manufacture  and  technical  uses  of 
non-ferrous  alloys  with  a  brief  theoretical  treatment. 

Practical  Alloying — Buchanan — 1910. — A  description  of  foundry 
practice  with  alloy  formulas.  Essentially  a  foundryman's 
handbook. 

Iron  and  Steel 

*Cast  Iron  in  the  Light  of  Recent  Research — Hatfield— 1912.— An 
extended  discussion  of  the  properties  of  cast  iron  with  many 
microphotographs.  Deals  with  shrinkage,  casting  temperature, 
growth,  etc. 


APPENDIX  141 

Crystallization  of  Iron  and  Steel— Mellor— 1905.— Six  lectures  on 
the  applications  of  metallography  in  engineering. 

Die  praktische  Nutzanwendung  der  Priifung  des  Eisens  mit  Hilfe 
des  Mikroskopes — Preuss — 1913. — A  brief  discussion  of  the  use 
of  the  microscope  in  the  study  of  good  and  of  defective  steel  and 
iron. 

Iron,  Steel  and  Other  Alloys— Howe — 1903. — One  of  the  earlier 
classics  on  the  metallography  of  iron  and  steel  by  the  dean  of 
American  metallographists. 

Metallographie  und  Warmebehandlung — Hanemann — 1915. — A  dis- 
cussion of  applied  metallography  and  heat  treatment  of  steel, 
designed  for  engineers. 

*  The  Metallography  and  Heat  Treatment  of  Iron  and  Steel — 

Sauveur — 1916. — A  detailed  treatment  of  the  metallography 
of  iron  and  steel  with  many  excellent  microphotographs.  One 
of  the  modern  standard  works  on  iron  and  stee? 
The  Metallography  of  Steel  and  Cast  Iron— Howe — 1916.— A  large 
book  dealing  with  many  theories  of  the  behavior  of  iron  and 
steel  and  especially  with  the  mechanism  of  plastic  deformation. 
Highly  speculative  and  of  interest  to  the  advanced  student. 

*  Microscopic    Examination    of    Steel — Fay — 1917. — Intended  for 

inspectors  of  steel  ordnance  materials.  A  very  brief  discussion 
of  methods  and  many  microphotographs  of  good  and  of  de- 
fective material. 

The  Microscopic  Analysis  of  Metals — Osmond — 1913  (Translated  by 
Stead). — An  English  edition  of  Osmond's  classic  in  the  field 
of  the  microphotography  of  steel  and  iron.  Includes  many  of 
Osmond's  remarkable  photographs  of  good  and  of  defective 
material. 

*  Physico-chemical  Properties  of  Steel — Edwards — 1916. — A  discus- 

sion of  the  chemical  and  structural  constitution  of  steel  and  the 
effects  of  heat  treatment  based  on  the  equilibrium  diagram. 
The  Sampling  and  Analysis  of  Iron  and  Steel — Bauer  and  Deiss — 
1912  (Translated  by  Hall  and  Williams)  .—Part  I  deals  with  the 
applications  of  metallography  to  the  selection  of  suitable  samples 
for  analysis. 

Heat  Treatment  of  Steel 

Heat  Treatment  of  Tool  Steel— Brearly— 1916.— Essentially  a  book 

for  the  tool  maker  but  gives  many  applications  of  metallography. 

Steel  and  Its  Heat  Treatment — Bullens — 1917. — A  detailed  discus- 


142  PRINCIPLES  OF  METALLOGRAPHY 

sion  of  the  heat  treatment  of  steel  and  the  uses  of  the  microscope 
in  following  the  changes  involved. 

Traitements  Thermiques  des  Produits  Me"tallurgiques — Guillefr — 
1909 — A  discussion  of  the  methods  and  results  of  quenching, 
tempering  and  annealing  of  alloys,  chiefly  iron  and  steel. 

Journals 

American  Institute  of  Metals — 1906-1918  (Incorporated  with  Am. 
Inst.  of  Min.  Eng.,  1919). — Deals  with  the  production  and 
metallography  of  non-ferrous  alloys. 

American  Institute  of  Mining  Engineers. — Includes  the  metallo- 
graphy of  iron  and  steel  and  since  Jan.,  1919  has  published 
the  non-ferrous  metallography  formerly  printed  in  Am.  Inst.  of 
Metals. 

American  Society  for  Testing  Materials — 1899  to  date. — In  the 
metals  section  has  published  much  information  on  the  physical 
properties  and  metallography  of  alloys. 

Bureau  of  Standards,  United  States. — Publishes  frequent  valuable 
monographs  on  various  branches  of  metallography. 

Journal  of  the  Iron  and  Steel  Institute  (British) — 1896  to  date.— 
Deals  with  the  preparation  and  metallography  of  iron  and  steel. 
Published  semi-annually. 

Journal  of  the  Institute  of  Metals  (British)— 1909  to  date.— A  semi- 
annual publication  of  the  Society  which  covers  for  non-ferrous 
alloys  the  field  occupied  by  the  Iron  and  Steel  Inst.  in  the  steel 
industry. 

Mitteilungen  aus  dem  Eisenhiittenschen  Instituts  Aachen — Wust — 
1906  to  date. — A  series  of  valuable  contributions  to  theoretical 
and  industrial  metallography. 

Zeitschrift  fur  Metallographie. — Published  original  articles  in  English, 
French,  Italian  and  German   and  included    abstracts    of  all 
metallographic  articles. 
In  addition  to  the  journals  devoted  primarily  to  the  properties  of 

metals  the  following  journals  frequently  publish  important  articles  on 

metallography. 
Chemical  and  Metallurgical  Engineering,  Metallurgie,  Revue  de 

M6tallurgie,  Stahl  und  Eisen,  Zeitschrift  fur  anorganische  Chemie. 


APPENDIX 
TABLE  3 


143 


The  Common  Industrial  Alloys. — The  composition  of  these  alloys 
varies  between  fairly  wide  limits  with  a  corresponding  variation  in 
physical  properties  and  applications. 

The  information  given  under  the  heading  "Uses"  is  intended 
merely  to  indicate  the  general  nature  of  the  alloy. 


*    Name 

Per  cent,  composition 

Uses 

Admiralty  metal  .... 
Aluminum  brass  .... 

Bronze, 
Aluminum  

Bearing 

Cu  70 
Zn  29 
Sn    1 
Cu  70-68 
Zn  27-31 
Al    1-3 

Cu   90,  Al  10 
Cu    70-90 

Condenser    tubes    for    use 
with  salt  water.     Marine 
fittings. 
Propeller      blades,    rudder 
frames,  sea  valves. 

Hard,    non-corrodible. 
Used  in  parts  exposed  to 
tanning,  sulphite  and  simi- 
lar corrosive  liquors. 
Bearings  of  various  sorts. 

Gear 

Sn     1-10 
Pb     0-15 
Zn    0-27 
Cu    89  Sn  11 

Used  for  heavy  gears  usu- 

Gun   

Cu    88.     Zn     10-8, 

ally  against  steel. 
Strong  valves  and  fittings. 

Phosphor  
Plastic 

Sn     2,  Pb  2 
Cu    80-77 
Sn    8-10 
Pb    9.5-15 
P      0-1.0 
Cu  70-50 

Bearing  metal,  wire,  rods, 
steam  fittings. 

Bearing  metal. 

Silicon 

Pb  30-50 
Ni  trace 
Cu  96,  Sn  4,  Si  tr. 

Telegraph  wires,   electrical 

Babbitt  metal  

Sn   70-90 

work. 
Bearings     and    antifriction 

"Genuine"  

Sb    7-24 
Cu  2-22 
Sn    88.9 
Sb    7.4 
Cu   3.5 

lining  for  bronze  bushings. 

144 


PRINCIPLES  OF  METALLOGRAPHY 


TABLE  3. — Continued 


Name 

Per  cent,  composition 

Uses 

Bell  metal  

Cu    80-75 

Bells,  gongs,  etc. 

Sn     20-25 

(Sometimes  Ag,   Ni 

or  other  metals) 

Brass, 
Gilding  metal  .  . 

Cu    99-80 

Cheap  jewellery,  gold  paint. 

Zn     1-20 

Dutch  metal 

Cu     80-76 

Thin   sheets    as   substitute 

Zn     20-24 

for  gold  leaf. 

Standard  

Cu     73-66 

Brass    for     cold     working. 

Zn     27-34 

Sheets,  tubes,  cartridges. 

White 

Cu  —  less  than  45 

Ornamental     castings     not 

requiring  strength. 

Brazing  metal  

Cu   85-Zn  15 

Britannia  metal  

Sn    95-90 

Cheap  table  ware. 

Sb    5-10 

Cu   1-3 

Chromel 

Ni    60 

Resistance  wire  for  heating 

(Nichrome)  

Cr    40 

units,  crucibles,  triangles. 

(approximate) 

tongs.     (Patented.) 

Cupro-nickel  

Cu   98-52 

Projectile     driving    bands, 

Ni   2-48 

rifle  bullet  caps,  electrical 

resistances. 

Constantan  

Cu   60. 

Used  with  copper  or  iron  to 

Ni    40  ' 

make  thermocouples. 

Delta  metal  

Cu  60 

See  Sterro  metal. 

Zn   40 

Mn  0.5-2 

Duralumin  

Al    95.5 

Strongest  and  best  of  alumi- 

Cu  3.0 

num  alloys.     Used  in  air- 

Mn 1.0 

plane  and   automobile 

Mg  0.5 

parts. 

Fusible  metals 

Lipowitz  

Bi    50 

These  and  other  ternary  and 

Pb  27 

quaternary      alloys       are 

Sn   13 

used    for    fuse    plugs    for 

Cd   10 

automatic  sprinklers. 

Woods  

Bi    38 

Pb  31 

Sn   15 

Cd  16 

APPENDIX 


145 


TABLE  3.— Continued 


Name 

Per  cent,  composition 

Uses 

German  silver  

See  nickel  silver. 

Gun  metal  

Cu  92-88 

Gears,  heavy  hydraulic  cast- 

Sn   8-12 

ings. 

Hercules  metal  Aluminum  brass  with 

Same    as   aluminum    brass 

Fe 

with  added  toughness. 

Invar  

Fe    64 

Low    coefficient   of   expan- 

Ni   36 

sion.     Used  in  clocks,  pre- 

cision instruments. 

Magnalium  

Al    90-94 

Scientific  instruments.    Bal- 

Mg 10-6 

ance  beams. 

Magnolia  metal             Pb  78 
(Lead  base  babbitt)    Sb    16 

Antifriction,  bearing  alloy. 

Sn   6 

Manganese  bronze-.  .    Cu  8S 

Propeller        blades.     Non- 

Sn   10 

corrodible  and  great  wear- 

Mn2 

ing  qualities. 

Manganin  Cu  82 

High    electrical    resistance 

!  Mn  15 

and  low  temperature  co- 

Ni   2.3 

efficient. 

Fe    0.6 

Monel  metal  •  Ni    72 

Almost   n  o  n-c  orrodible. 

Cu  26.5 

Used  for  propeller  blades, 

Fe    1.5 

wire,  sheets,  etc. 

Muntz  metal  <  Cu   60 

Sheathing  for  ships,    bolts, 

Zn   40 

nuts,  condenser  tubes. 

Naval  brass  Cu  62 

Properties    like    Muntz 

Zn    37 

metal.     Less    easily     cor- 

Sn    1 

roded  by  sea  water. 

Nickeliu  

Cu    74.5 

Resistance  wire. 

Ni    25 

Fe    0.5 

Nickel  silver                  Ni    18-25 

Table  ware,  cheap  jewellery, 

(German  silver)  .  .  .    Zn   20-30 

base  for  silver  plating. 

Cu  (Remainder) 

Palau           

Pd 

Substitute  for  platinum  in 

Au 

chemical  crucibles,  dishes, 

etc.     (Patented.) 

Pewter            .    . 

Sn    85-90 

Platters,    bowls,  cups,  etc. 

Sb    15-10 

Little  used  at  present. 

Platinoid  Cu    60 

High    resistance    wire    but 

Zn    24 

not    suitable    for    heating 

Ni    14 

coils. 

W  1-2 

146 


PRINCIPLES  OF  METALLOGRAPHY 


TABLE  3.— Continued 


Name 

Per  cent,  composition 

Uses 

• 
Platinite 

Fe54 

Same  coefficient  of  expan- 

Ni46 

sion    as    glass.     Used    as 

C  0.15 

substitute  for  platinum  in 

equipping       incandescent 

lamps. 

Platinum  Iridium  .  .  . 

Pt  90 

Standard  meter  and  other 

Ir   10 

standards.       Thermo- 

couple with  platinum. 

Rheotan  

Cu52 

High    resistance    but    not 

Zn  18 

suitable  for  heating  coils. 

Ni25 

Fe5 

Shot  metal  

Pb99 

Casting  bullets   and   small 

As  1 

shot. 

Solder 

Soft          

Pb  67,  Sn  33 

Plumbers  solder. 

Medium  

Pb  50,  Sn  50 

Hard  

Pb  33,  Sn  67 

Speculum  metal.  .  .  . 

Cu  70-65 

Takes    a   high   polish. 

Sn  30-35 

Formerly  used  in  reflectors 

for  telescopes. 

Steel 

Plain   Carbon.  .  . 

C    0.05-0.15 

Boiler   plate,   rivets,   sheet 

steel,  case  hardening  stock. 

C    0.15-0.25 

Structural    work     bridges, 

shafting. 

C    0.25-0.40 

Axles,  connecting  rods,  pis- 

ton rods. 

C    0.4-0.75 

Rails,  steel  castings. 

C    0.6-0.8 

Cutlery,    wood    working 

tools,  drills. 

C    0.8-1.0 

Springs,  lathe  tools,   drills. 

C    1.0-1.2 

Large     lathe     tools,     axes, 

knives. 

C    1.2-1.5 

Saws,  files,  balls  for  bear- 

ings, razors. 

Chrome  

Cr  less  than  3 

Projectiles,  files. 

Chrome-tungsten 

C    0.25-1 

High  speed  tools.     May  be 

W  5-25 

run  at  500°-600°C.  without 

Cr  2-10 

losing  their  edge. 

Vd  0.25-1 

APPENDIX 
TABLE  3. — Continued 


147 


Name 

Per  cent,  composition 

Uses 

Steel  (continued) 

"Chrome- 

C     0.25-1 

Gears  and  springs. 

vanadiurn 

Cr   0.8-1.1 

Vd  0.15 

Manganese  

Mn  6-15 

Used     on     sharp    railroad 

curves,  frogs,  switches,  etc., 

where  wear  is  hard. 

Nickel 

Ni    3-4 

Drive  shafts    crank  shafts, 

gears  and  other  automo- 

bile parts. 

Nickel- 

Ni    1-4 

Armor  plate 

chromium  .  .  . 

Cr    0.45-2 

Silicon  

Si,  less  than  5 

Has  high  permeability  and 

low    hysteresis.     Used   in 

dynamo  construction. 

Stellite  

Co   80-50 

Non-corrodible.     Used      in 

Cr    20-50 

cutlery,     surgical     instru- 

ments.    (Patented.) 

Sterro  metal  

Cu   60 

Strong  as  mild  steel  and  not 

(Aich's  metal  Delta 

Zn  38 

easily  corroded.     Used  in 

metal)  :  

Fe   2 

hydraulic    cylinders,     sea 

water  valves. 

Type  metal  

Pb  60-85 

Sb    8-20 

Sn     5-35' 

. 

148 


PRINCIPLES  OF  METALLOGRAPHY 


TABLE  4. 
Temperature  Conversion  Table  (Condensed) 

Degrees  Centigrade  to  Degrees  Fahrenheit. 
Degrees  Fahrenheit  =  %  Degrees  Centigrade  +  32°. 
Degrees  Centigrade  =  %    (Degrees  Fahrenheit  -  32°). 


Decrees—* 
Centigrade 

0      10 

20 

30 

40 

50 

60 

70 

80 

90 

Degrees  Fahrenheit 

0 

32 

50 

68 

86 

104 

122 

140 

158 

176 

194 

100 

212 

230 

248 

266 

284 

302 

320 

338 

356 

374 

200 

392 

410 

428 

446 

464 

482 

500 

518 

536 

554 

300 

572 

590 

608 

626 

644 

662 

680 

698 

716 

734 

400 

752 

770 

788 

806 

824 

842 

860 

878 

896 

914 

500 

932 

950 

968 

986 

1004 

1022 

1040  1058 

1076 

1094 

600 

1112 

1130 

1148 

1166 

1184 

1202 

1220 

1238  I  1256 

1274 

700 

1292 

1310 

1328 

1346 

1364   1382 

1400 

1418 

1436 

1454 

800 

1472 

1490 

1508 

1526 

1544 

1562 

1580 

1598 

1616]  1634 

900 

1652 

1670 

1688 

1706 

1724 

1742 

1760 

1778 

17961  1814 

1000 

1832 

1850 

1868 

1886 

1904  !  1922   1940  j  1958 

1976  1994 

1100 

2012 

2030 

2048 

2066 

2084   2102  !  2120  2138 

2156  2174 

1200 

2192 

2210 

2228 

2246 

2264 

2282   2300  2318 

2336  2354 

1300 

2372 

2390 

2408 

2426 

2444 

2462   2480  2498 

2516 

2534 

1400 

2552 

2570 

2588 

2606 

2624 

2642  !  2660  |  2678 

2696 

2714 

1500 

2732 

2750 

2768 

2786 

2804   2822  1  2840 

2858 

2876 

2894 

1600 

2912 

2930 

2948 

2966 

2984 

3002  j  3020  3038 

3056 

3074 

1700 

3092 

3110 

3128 

3146 

3164 

3182   3200  3218 

3236 

3254 

1800 

3272 

3290 

3308 

3326 

3344 

3362  I  3380  ;  3398 

3416 

3434 

1900 

3452 

3470 

3488 

3506 

3524 

3542 

3560 

3578 

3596 

3614 

2000 

3632 

3650 

3668 

3686 

3704 

3722 

3740 

3758 

3776 

3794 

APPENDIX 


149 


TABLE   5. 

Melting  Points  and  Atomic  Weights  of  the  More  Important 
Metals  and  Metalloids 


Element 


Symbol 


|      weight     j 


Melting  point,  deg.  C. 


Aluminum  
Antimony  
Arsenic  
Barium  

Al 
Sb 
As 
Ba 

27.1 
120.2 
75.0 
137.4 

658.7 
630.5 
850.0  (?) 
850.0 

Beryllium 

Be 

9  1 

1278  0 

Bismuth  
Boron  
Cadmium  

Bi 
B 

Cd 

208.0 
11.0 
112.4 

271.0 
2000.0-2500.0  (?) 
320.9 

Calcium  

Ca 

40.1 

800.  Oca. 

Carbon 

C  (Diamond) 

12  0 

>  3600.0 

Cerium 

Ce 

140  25 

>  800.0 

Caesium  
Cobalt  
Chromium  
Copper  
Gallium  
Gold  
Indium  
Iridium  

Cs 
Co 
Cr 
Cu 
Ga 
Au 
In 
Ir 

132.9 
59.0 
52.1 
63.6 
70.0 
197.2 
115.0 
193.0 

26.0 
1480.0 
1520.0 
1084.1 
30.0 
1063.5 
155.0 
2350.0  (?) 

Iron  

Fe 

55.9 

1530.0 

Lanthanum 

La 

138  9 

810.0  (?) 

Lead 

Pb 

206.9 

327.4 

Lithium  
Magnesium  
Manganese  
Mercury 

Li 
Mg 

Mn 
Hg 

7.03 
24.36 
55.0 
200  0 

186.0 
635.0 
1260.0 
-38.9 

Molybdenum  
Nickel  
Osmium  

Mo 

Ni 
Os 

96.0 

58.7 
191.0 

2500.0  (?) 
1451.0 
2700.0  (?) 

Palladium 

Pd 

106.5 

1549.0 

Phosphorus 

P 

31.0 

I.  44-11.  930 

Platinum  

Pt 

194.8 

1780.0 

K 

39  15 

62.5 

Rubidium 

Rb 

85.5 

38.0 

Ruthenium  
Selenium  

Ru 

Se 
Si 

101.7 
79.2 

28.4 

2450.0(7) 
217.0 
1420.0 

Silver 

Ag 

107.93 

961.5 

Sodium  

Na 

23.5 

97.5 

150 


PRINCIPLES  OF  METALLOGRAPHY 


TABLE  5.— Continued 


Element                              Symbol 

Atomic 
weight 

Melting  point,  deg.  C. 

Sulphur  S 
Strontium  Sr 

32.6 
87.6 

f     I.  112.8-II.  119.2 
\     III.  106.2 
>Ca,<  Ba(?) 

Thallium  Tl 

204.1 

302.0 

Tellurium                                      Te 

127  6 

450  0 

Tin  Sn 
Titanium  Ti 
Tungsten  W 
Uranium  .              Ur 
Vanadium  V 

119.0 
48.1 
184.0 
238.5 
51.2 

232.0 
1800.0 
>3000.0 
<  1850.0 
1720.0 

Zinc                                                Zn 

65.4     1 

419  0 

INDEX 


Abrasives 30 

methods  of  use  of 31 

Admiralty  metal 92 

Alloy  steels 113 

diagram  for 114 

Alloys,  burning  of 133 

definition  of 1 

eutectic 9 

mechanical  hardening  of 72 

mechanical  testing  of 138 

microscopical  examination  of 29 

overheating  of 133 

oxidation  of • 22 

preparation  of,  for  examination 29 

stirring  of,  during  melting 21 

ternary 58 

two-layer 3 

Aluminum,  alloys  of 69 

effect  of,  on  brass  .    .    .    .' 92 

preparation  of,  for  examination 70 

Aluminum  bronze • 92 

Aluminum-lead  alloys 6 

Amorphous  binding  material 76 

Amorphous  cement  theory 73 

Annealing,  defective 133 

reasons  for .    .   y, .    .    .  116 

temperature  of 116 

Antimony-copper  alloys 58 

Antimony-lead  alloys 10 

structure  of 14 

uses  of 16 

Atomic  per  cent.,  calculation  of 23 

use  of 22 

Atomic  weights,  table  of 149 

Austenite 108 

151 


152  INDEX 


Babbitt  metal 64 

casting  of 65, 133 

segregation  in 128 

Bell  metal 81 

Bismuth-tin  alloys 15 

Books  on  metallography,  list  of 138 

Brass 82 

a-form 86 

annealing  of 88 

0-form 84 

cold-working  of 83 

experiments  with 136 

jewelry 83 

solder 86 

twinning  of 86 

white \ 86 

Bronze 77 

aluminum • 92 

bearing 79 

coinage 78 

diagram  for '  -    -    •    •  78 

experiments  with 137 

gear 78,79 

Government 79 

manganese • 92 

phosphor 79 

plastic &• 

C 

Carbon,  effect  of,  on  steel 105 

Case  hardening 117 

materials  used  for 117 

temperatures  for.    .  '.    .    ..." 

Cementation 117 

Cementite 102 

Kourbatoff's  reagent  for 104 

spheroidized •    •  107 

Changes  in  solid  state 56 

Chemical  composition,  effect  of •  123 

Chromel 49 

Compounds,  intermetallic 

Concealed  maximum 52 

diagram  of 53 


INDEX  153 

Conductivity,  effect  of  solid  solutions  on 71 

Constantan 49 

use  of,  in  thermocouple 24 

Containers  for  melted  metals    .    .    . 20 

Cooling,  rate  of .    . 25 

Cooling  curve,  form  of , 1,  2 

plotting  of 28 

Copper 70 

deoxidizing  of 72 

mechanical  hardening  of 72 

properties  of 72 

Copper  alloys,  etching  of 76 

Copper-manganese  alloys 43 

diagram  of  .    . 44 

uses  of 48 

Copper-nickel  alloys 48 

diagram  of 48 

uses  of 49 

Copper-silver  alloys 40 

diagram  of .'    . 40 

uses  of .   42 

Critical  points 96 

Critical  range ....  96 

D 

Diagrams,  equilibrium ....  4 

experimental,  construction  of .  .  27,  28 

freezing  point .  4 

interpretation  of ...  7, 39 

Duralumin 69 

heat  treatment  of 70 

Dutch  metal .  83 

E 

Etching 31 

of  copper  alloys 76 

of  steel .  ....  .  .  ...  ...  .  ......... 103 

reagents  for .  ...  .  .,...-....-.- 32 

Eutectic .......... 11 

alloy 11 

location  of,  by  time  lines 13 

point 11 

temperature 11 

Eutectoid     .                                                              57 


154  INDEX 

F 

Ferrite 102 

Furnaces 18, 19 

Fuse  plugs 65 

G 

German  silver 64 

Gilding  metal 83 

Gold,  white 49 

Grain  size '. 88 

factors  for 90 

formulas  for 90 

measurement  of 88 

Gun  metal 78 

H 

Heat  treatment  of  steel 115 

books  on 141 

High  speed  tools 115 

"Hold,  "definition  of 12 

Hydrogen,  use  of  in  melting  alloys 22 

I 

Industrial  alloys. 143 

books  on 140 

composition  of 143 

uses  of 143 

Intermetallic  compounds 49 

books  on 140 

hardness  of 55 

uses  of 55 

Iron,  allotropic  forms  of 96 

cast 118 

critical  points  in 96 

experiments  with 138 

gray 120 

malleable 120 

mottled 122 

white 118 

wrought 97 

Iron  and  steel,  books  on    .                                             140 


INDEX  155 

J 

Journals,  list  of 141 

K 

Kourbatoff 's  reagent  for  cementite 104 

L 

Laboratory  course  in  metallography 136 

Laboratory  methods  of  metallography 18 

Lead,  effect  of,  on  brass 92 

Lead-tin  alloys 16 

diagram  of 16 

uses  of 16 

Liquidus 11 

curve 45 

M 

Magnesium-tin  alloys 51 

Malleabilizing 120 

Manganese  bronze 92 

Manganese  steel . 114 

Manganin 48 

Martensite 108 

Maximum,  concealed : 52 

open 50 

Melting,  methods  of 27 

Melting  points,  table  of 149 

Metallography,  books  on 138 

definition  of 1 

laboratory  course  in 136 

laboratory  methods  of 18 

Metals,  atomic  weights  of 149 

melting  points  of 149 

Microscope,  metallographic 34 

vertical  illuminator  for 33 

Millivoltmeter,  calibration  of 25 

types  of 26 

use  of 24 

Monel  metal 49 

Mounting  photographic  prints 36 

cards  for 37 

Muntz  metal 84 

segregation  in 128 


156  INDEX 

N 

Nichrome '•.'•• 

Nickel  silver 

Non-ferrous  alloys ....     69 

O 

Open  maximum 

•  diagram  of 

with  solid  solutions 

Overheating,  definition  of I33 

P 

Palau 49 

Pearlite 

Phase  rule 

definition  of  terms 

diagram  illustrating ....     67 

uses  of 67 

Phosphide  in  steel  .    .    . 
Plastic  bronze 

segregation  in  .    .    .    .    - 
Plates,  photographic 

development  of 

exposure  of  . 

kinds  of 

partial  exposure  of 

Polishing «    •    .    • 

by  hand 30 

mechanical 

Potentiometer,  use  of 

Printing,  photographic 

Prints,  mounting  of ....     37 

Pyrometers 

Q 

Quenching 115 

incorrect *3 

S 

Season  cracks 91,130 

test  for 91 


INDEX  157 

Segregation 124 

in  brass  and  bronze 124 

in  Muntz  metal 128 

in  plastic  bronze 128 

in  steel 125 

Slag  in  iron 125 

Slip  bands ~.    .    .    .  75 

Solder,  brass 86 

plumber's 16 

tin 16 

Solid  solution      39 

curve  of 46 

development  of 42,  43 

effect  of,  on  conductivity 71 

freezing  of 43 

microscopic  appearance  of '.  47 

Solidus 12 

curve 45 

Sorbite Ill 

production  of 112 

Specimens,  preserving  of 32 

Spheroidizing 107 

Steel 95 

alloy 113 

annealing  of 115 

case  hardening  of 117 

chrome-tungsten :    .    .    .  115 

cold  working  of 130 

composition  of 98 

diagram  for 99 

etching  of 103 

experiments  with 137 

heat  treatment  of 115 

high  speed  tool 115 

hot  working  of 130 

hyper-eutectoid '.,.;....    ....    ....  101 

hypo-eutectoid 101 

incomplete  transformations  in 101 

manganese '. 114 

metallographic  constituents  of .    .    .    .  V  .    .  ...    102,  112 

phosphide  in 128 

quenching  of •    108,  115 

sulphide  in 

tempering  of 108,  115 

uses  of 107 

Stellite.                                       49 


158  INDEX 

Strain  hardness 76 

Sulphides  in  steel 125 

detection  of,  by  sulphur  prints 126 

Surrounding,  appearance  of 55 

cause  of    .  .54 


Temperature,  conversion  table  for .  148 

measurement  of 23 

Tempering 115 

incorrect  temperature  for 134 

Ternary  alloys 58 

binary  eutectics  in 63 

binary  surfaces  of 61 

changes  occurring  in 63 

composition  of 60 

diagram  of,  with  contour  lines 62 

microscopic  appearance  of 64 

space  model  of 59 

solidus  surface  of 63 

uses  of 64 

Testing,  mechanical  ........... 138 

Thermal  junction 24 

Thermic  analysis 4 

Thermoelement 24 

Thermos  bottle,  use  of,  for  cold  junction 24 

Time  curve      4 

construction  of 4 

uses  of      6,13 

Tin,  effect  of,  on  brass  . 92 

Transition 54 

Troostite      109 

production  of 110 

W 

Weight  per  cent.,  conversion  to  atomic  ......    t 23 

Wood's  metal                                                                                           .  65 


1  24 


UNIVERSITY  OF  CALIFORNIA,  LOS  ANGELES 

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MAR  3  - 


DEC  1  «  1953 


AUG     6  1959 
AUG  2  7  1959 


2  «  1976 


P    519 


THE  L 

UNWERStl  ,   O..         Uf 
LOS  ANGELES 


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;  SOUTHERN  REGIONAL  UBRARY  FACILITY 


A    001  187889     9 


